Rescue from errors to prevent long error messages

[julie-ng]

For example:

We already output file not found… or is this another bug?

julieng@air in project on dev/events-1
$ a send event-comment-attendees-host -s 'Event Comments (to host)'
   warning  No build found for 'event-activity-comment-attendees-host'. Building now...
/Users/julieng/Projects/antwort/generator/lib/antwort/builder/builder.rb:24:in `initialize': undefined method `to_sym' for nil:NilClass (NoMethodError)
    from /Users/julieng/Projects/antwort/generator/lib/antwort/cli/cli.rb:123:in `new'
    from /Users/julieng/Projects/antwort/generator/lib/antwort/cli/cli.rb:123:in `build'
    from /Users/julieng/Projects/antwort/generator/lib/antwort/cli/cli.rb:84:in `send'
    from /Users/julieng/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
    from /Users/julieng/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
    from /Users/julieng/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
    from /Users/julieng/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
    from /Users/julieng/Projects/antwort/generator/bin/antwort:5:in `<top (required)>'
    from ./bin/antwort:16:in `load'
    from ./bin/antwort:16:in `<main>'

[julie-ng]

Another example:

julieng@air in internations on dev/events-1
$ a b past-event-notification-bundle
Error: Template 'past-event-notification-bundle' not found.
/Users/julieng/Projects/antwort/generator/lib/antwort/builder/email.rb:27:in `exist?': no implicit conversion of nil into String (TypeError)
    from /Users/julieng/Projects/antwort/generator/lib/antwort/builder/email.rb:27:in `build'
    from /Users/julieng/Projects/antwort/generator/lib/antwort/cli/cli.rb:124:in `build'
    from /Users/julieng/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
    from /Users/julieng/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
    from /Users/julieng/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
    from /Users/julieng/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
    from /Users/julieng/Projects/antwort/generator/bin/antwort:5:in `<top (required)>'
    from ./bin/antwort:16:in `load'
    from ./bin/antwort:16:in `<main>'