Improve a couple exception messages related to variants and mime types

Avoid one-liner conditionals when they are too big. Avoid concatenating
strings to build error messages. Improve messages a bit.

actionpack/lib/abstract_controller/collector.rb

@@ -23,15 +23,17 @@ def #{sym}(*args, &block)                # def html(*args, &block)
   protected
 
     def method_missing(symbol, &block)
-      mime_const = symbol.upcase
-
-      raise NoMethodError, "To respond to a custom format, register it as a MIME type first:" +
-                           "http://guides.rubyonrails.org/action_controller_overview.html#restful-downloads." +
-                           "If you meant to respond to a variant like :tablet or :phone, not a custom format," +
-                           "be sure to nest your variant response within a format response: format.html" +
-                           "{ |html| html.tablet { ..." unless Mime.const_defined?(mime_const)
+      const_name = symbol.upcase
+
+      unless Mime.const_defined?(const_name)
+        raise NoMethodError, "To respond to a custom format, register it as a MIME type first: " \
+          "http://guides.rubyonrails.org/action_controller_overview.html#restful-downloads. " \
+          "If you meant to respond to a variant like :tablet or :phone, not a custom format, " \
+          "be sure to nest your variant response within a format response: " \
+          "format.html { |html| html.tablet { ... } }"
+      end
 
-      mime_constant = Mime.const_get(mime_const)
+      mime_constant = Mime.const_get(const_name)
 
       if Mime::SET.include?(mime_constant)
         AbstractController::Collector.generate_method_for_mime(mime_constant)

actionpack/lib/action_controller/metal/mime_responds.rb

@@ -348,8 +348,10 @@ def respond_to(*mimes, &block)
     # 2. <tt>:action</tt> - overwrites the default render action used after an
     #    unsuccessful html +post+ request.
     def respond_with(*resources, &block)
-      raise "In order to use respond_with, first you need to declare the formats your " \
-            "controller responds to in the class level" if self.class.mimes_for_respond_to.empty?
+      if self.class.mimes_for_respond_to.empty?
+        raise "In order to use respond_with, first you need to declare the " \
+          "formats your controller responds to in the class level."
+      end
 
       if collector = retrieve_collector_from_mimes(&block)
         options = resources.size == 1 ? {} : resources.extract_options!

actionpack/lib/action_dispatch/http/mime_negotiation.rb

@@ -66,15 +66,15 @@ def formats
           end
       end
 
-      # Sets the \variant for template
+      # Sets the \variant for template.
       def variant=(variant)
         if variant.is_a? Symbol
           @variant = variant
         else
-          raise ArgumentError, "request.variant must be set to a Symbol, not a #{variant.class}. For security reasons," +
-                               "never directly set the variant to a user-provided value, like params[:variant].to_sym." +
-                               "Check user-provided value against a whitelist first, then set the variant:"+
-                               "request.variant = :tablet if params[:some_param] == 'tablet'"
+          raise ArgumentError, "request.variant must be set to a Symbol, not a #{variant.class}. " \
+            "For security reasons, never directly set the variant to a user-provided value, " \
+            "like params[:variant].to_sym. Check user-provided value against a whitelist first, " \
+            "then set the variant: request.variant = :tablet if params[:variant] == 'tablet'"
         end
       end