Create better error message if source branch of pull request is deleted

Currently, if the source branch of a pull request is deleted, the pull request can end up in a bad state where you're unable to edit the pull request (source branch will appear missing). It will make more sense to warn the user that the source branch has been deleted instead of trying to load the pull request indefinitely.

...and is not possible change the source branch. The field appears empty, when I open the field (clicking it) and search for another branch, no results is presented.