Improve error message when attempting to reopen MR and there's a new open MR for the same branch

Summary

We should display a meaningful error message when you attempt to reopen a merge request and there is a new open merge request for the same branch.

Steps to reproduce

1. Create a new MR with branch foo => A
2. Close A
3. Open a new MR with branch foo => B
4. Attempt to reopen A