Return 40x error codes if branch could not be deleted in UI

What does this MR do?

This MR makes BranchesController return the proper status code in case of a failure to delete a branch.

Why was this MR needed?

Deleting a branch would always return status 200, even if the branch could not be found or deleted for some reason. This was confusing because while trying to debug an issue with encoded slashes, it looked like the deletion was successful when it had failed.

What are the relevant issue numbers?

This issue was uncovered in #1804 (closed)