API: extracted helper method to provide 400 bad request error with description

Extracted a method for 400 error (Bad request) and adjusted code accordingly. The name of
the missing attribute is used to show which one was missing from the request. It is used to
give an appropriate message in the json response.

spec/requests/api/merge_requests_spec.rb

@@ -32,6 +32,11 @@ describe Gitlab::API do
       response.status.should == 200
       json_response['title'].should == merge_request.title
     end
+
+    it "should return a 404 error if merge_request_id not found" do
+      get api("/projects/#{project.id}/merge_request/999", user)
+      response.status.should == 404
+    end
   end
 
   describe "POST /projects/:id/merge_requests" do