Reopening a MR should throw a reasonable error if MR for branch already exits

Summary

When reopening a closed MR while a new MR for the original branch exists the interface shows no error but silently fails.

Steps to reproduce

1. Create an new branch
2. Commit changes to branch
3. Create new MR_1 from branch
4. Close MR_1
5. Create a new MR_2 from the same branch
6. Try to reopen MR_1

See MR_1 and MR_2 in smark/MR-test

What is the current bug behavior?

Nothing happens but a reload of the site.

What is the expected correct behavior?

An error should be shown describing why the MR could not be reopenend. Just like the error when trying to create a new MR_2 when MR_1 alread exists.

Relevant logs and/or screenshots

n/a

Output of checks

This bug happens on GitLab.com.