configuration.rb, manifest.rb to handle no compile

This fix addresses the cases where compile is disabled:

1. If React on Rails depends on Webpacker, but the React on Rails
installer has not yet generated a webpacker.yml file or a React on Rails
installation is not using Webpacker, then the gem loading of Webpacker
will crash due to the missing config file.

2. If React on Rails has its own helper to compile assets for tests,
then the manifest might need to get reloaded after Webpacker statically
loads.

lib/webpacker/configuration.rb

@@ -47,15 +47,22 @@ def fetch(key)
     end
 
     def data
-      if env.development?
-        refresh
-      else
+      if env.production?
         @data ||= load
+      else
+        refresh
       end
     end
 
     def load
-      YAML.load(config_path.read)[env].deep_symbolize_keys
+      if config_path.exist? &&
+        (@parsed_mtime.nil? ||
+          ((config_mtime = File.mtime(config.public_manifest_path)) > @parsed_mtime))
+        @parsed_mtime = config_mtime
+        YAML.load(config_path.read)[env].deep_symbolize_keys
+      else
+        {}
+      end
     end
 
     def defaults

lib/webpacker/manifest.rb

@@ -42,19 +42,36 @@ def find(name)
 
     def handle_missing_entry(name)
       raise Webpacker::Manifest::MissingEntryError,
-        "Can't find #{name} in #{config.public_manifest_path}. Is webpack still compiling?"
+        "Can't find #{name} in #{config.public_manifest_path}. Manifest contains: #{@data}"
+    end
+
+    def missing_file_from_manifest_error(bundle_name)
+      msg = <<-MSG
+Webpacker can't find #{bundle_name} in #{config.public_manifest_path}. Possible causes:
+1. You are hot reloading.
+2. You want to set Configuration.compile to true for your environment.
+3. Webpack has not re-run to reflect updates.
+4. You have misconfigured Webpacker's config/webpacker.yml file.
+5. Your Webpack configuration is not creating a manifest.
+Your manifest contains:
+#{@data.to_json}
+    MSG
+      raise(Webpacker::FileLoader::NotFoundError.new(msg))
     end
 
     def data
-      if env.development?
-        refresh
-      else
+      if env.production?
         @data ||= load
+      else
+        refresh
       end
     end
 
     def load
-      if config.public_manifest_path.exist?
+      if config.public_manifest_path.exist? &&
+        (@parsed_mtime.nil? ||
+          ((manifest_mtime = File.mtime(config.public_manifest_path)) > @parsed_mtime))
+        @parsed_mtime = manifest_mtime
         JSON.parse config.public_manifest_path.read
       else
         {}