Improve error message when creating a new MR in an existent branch through the UI

Summary

The error message when creating a new MR in an existent branch through the UI is not very helpful

How do I switch to a branch from this view?

Steps to reproduce

1. Edit a file
2. Change the target branch to a new one
3. Click commit changes
4. Don't create a new MR
5. Go back to the edit page
6. Edit some more
7. Try to commit to the same branch

What is the current bug behavior?

Although the message is correct it is not very helpful for the end user since in that page there is no way to go that branch

What is the expected correct behavior?

1. Give the user a link to that branch with the edited file changes.