Merge request error display

Fixes #8432 

Currently, GitLab displays an error message whenever a new merge request is created. This is often confusing for users. There is another catch point that will show an error (see screenshot) if a user clicks "Compare branches" without specifying a source or target. If a source and target branch are not specified under other circumstances I think it's OK to quietly 'fail' without showing a message. I'm not sure if this is acceptable but I can't see any pitfalls so far. Please provide feedback.

Before:
![screen shot 2014-12-26 at 3 28 35 pm](https://cloud.githubusercontent.com/assets/2394772/5559464/e7084d6e-8d13-11e4-97f3-d0b811b716ad.png)

Existing and unchanged (if user clicks 'Compare branches' without specifying'):
![screen shot 2014-12-26 at 3 25 03 pm](https://cloud.githubusercontent.com/assets/2394772/5559461/c0e63a06-8d13-11e4-94b9-b93ced7fba8f.png)