Add validation errors to Merge Request form

What does this MR do?

Adds validation errors when a user attempts to create Merge Request with a branch that doesn't exist.

Are there points in the code the reviewer needs to double check?

All the work is done in app/services/merge_requests/build_service.rb. I did change the method signature for #build_failed but it is backwards compatible.

Why was this MR needed?

Previously, this case caused a 500 response code.

Screenshots (if relevant)

http://screencast.com/t/Y4YPMBRF5F

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #22448 (closed)