Add placeholder validation target to error message

[joker1007]

In some case, placeholder_validate! checks composited value.
For example, fluent-plugins-nursery/fluent-plugin-bigquery#115.

this patch enable user to can find wrong config easily by error message even if In the case.

Output Example

% be fluentd -c fluent.conf -v
2017-03-10 23:04:32 +0900 [info]: fluent/supervisor.rb:696:read_config: reading config file path="fluent.conf"
2017-03-10 23:04:32 +0900 [info]: fluent/supervisor.rb:537:supervise: starting fluentd-0.14.13 pid=26713
2017-03-10 23:04:32 +0900 [info]: fluent/supervisor.rb:546:supervise: spawn command to main:  cmdline=["/home/joker/.rbenv/versions/2.3.3/bin/ruby", "-Eascii-8bit:ascii-8bit", "-rbundler/setup", "/home/joker/fluentd_workspace/.bundle/ruby/2.3.0/bin/fluentd", "-c", "fluent.conf", "-v", "--under-supervisor"]
2017-03-10 23:04:32 +0900 [info]: fluent/engine.rb:114:block in configure: gem 'fluentd' version '0.14.13'
2017-03-10 23:04:32 +0900 [info]: fluent/engine.rb:114:block in configure: gem 'fluent-mixin-config-placeholders' version '0.4.0'
2017-03-10 23:04:32 +0900 [info]: fluent/engine.rb:114:block in configure: gem 'fluent-mixin-plaintextformatter' version '0.2.6'
2017-03-10 23:04:32 +0900 [info]: fluent/engine.rb:114:block in configure: gem 'fluent-mixin-rewrite-tag-name' version '0.1.0'
2017-03-10 23:04:32 +0900 [info]: fluent/engine.rb:114:block in configure: gem 'fluent-plugin-bigquery' version '0.5.0.beta'
2017-03-10 23:04:32 +0900 [info]: fluent/engine.rb:114:block in configure: gem 'fluent-plugin-record-modifier' version '0.5.0'
2017-03-10 23:04:32 +0900 [info]: fluent/engine.rb:114:block in configure: gem 'fluent-plugin-remote_syslog' version '0.3.2'
2017-03-10 23:04:32 +0900 [info]: fluent/agent.rb:124:add_match: adding match in @FLUENT_LOG pattern="fluent.info" type="remote_syslog"
2017-03-10 23:04:32 +0900 [error]: #0 fluent/supervisor.rb:665:block in main_process: config error file="fluent.conf" error_class=Fluent::ConfigError error="Parameter 'remote_syslog: host=logs2.papertrailapp.com/host_with_port=/facility=user/severity=${fugafuga}/program=fluentd.repro-local-test' doesn't have tag placeholder"
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/plugin/output.rb:606:in `validate_tag!'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/plugin/output.rb:581:in `validate!'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/plugin/output.rb:530:in `block in placeholder_validate!'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/plugin/output.rb:529:in `each'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/plugin/output.rb:529:in `placeholder_validate!'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/reproio/fluent-plugin-remote_syslog/lib/fluent/plugin/out_remote_syslog.rb:53:in `configure'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/plugin.rb:164:in `configure'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/agent.rb:128:in `add_match'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/agent.rb:71:in `block in configure'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/agent.rb:64:in `each'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/agent.rb:64:in `configure'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/label.rb:31:in `configure'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/root_agent.rb:83:in `block in configure'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/root_agent.rb:83:in `each'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/root_agent.rb:83:in `configure'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/engine.rb:117:in `configure'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/engine.rb:91:in `run_configure'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/supervisor.rb:734:in `run_configure'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/supervisor.rb:503:in `block in run_worker'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/supervisor.rb:662:in `main_process'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/supervisor.rb:499:in `run_worker'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/lib/fluent/command/fluentd.rb:300:in `<top (required)>'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/bin/fluentd:5:in `require'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/.ghq/github.com/fluent/fluentd/bin/fluentd:5:in `<top (required)>'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/fluentd_workspace/.bundle/ruby/2.3.0/bin/fluentd:22:in `load'
  2017-03-10 23:04:32 +0900 [debug]: #0 fluent/supervisor.rb:661:main_process: /home/joker/fluentd_workspace/.bundle/ruby/2.3.0/bin/fluentd:22:in `<main>'
2017-03-10 23:04:33 +0900 [info]: serverengine/multi_process_server.rb:136:alive?: Worker 0 finished unexpectedly with status 2
2017-03-10 23:04:33 +0900 [info]: serverengine/server.rb:51:stop: Received graceful stop

[repeatedly]

Good. Thx!