Include Cocaine error when there was an error processing thumbnails (#2415)

lib/paperclip/thumbnail.rb

@@ -85,7 +85,10 @@ module Paperclip
           dest: File.expand_path(dst.path),
         )
       rescue Terrapin::ExitStatusError => e
-        raise Paperclip::Error, "There was an error processing the thumbnail for #{@basename}" if @whiny
+        if @whiny
+          message = "There was an error processing the thumbnail for #{@basename}:\n" + e.message
+          raise Paperclip::Error, message
+        end
       rescue Terrapin::CommandNotFoundError => e
         raise Paperclip::Errors::CommandNotFoundError.new("Could not run the `convert` command. Please install ImageMagick.")
       end

spec/paperclip/thumbnail_spec.rb

@@ -179,10 +179,12 @@ describe Paperclip::Thumbnail do
         end
 
         it "errors when trying to create the thumbnail" do
-          assert_raises(Paperclip::Error) do
-            silence_stream(STDERR) do
+          silence_stream(STDERR) do
+            expect {
               @thumb.make
-            end
+            }.to raise_error(
+              Paperclip::Error, /unrecognized option `-this-aint-no-option'/
+            )
           end
         end