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

See merge request !823

Add ref switching support for graphs

It is now possible to view contributor and commit statistics for
refs beyond the default branch. While the Git ref already was part of
the path it has not been used before.

Additionally, the code in RefsController#switch has been slighliy
refactored since it would have become confusing with additional if-else
branches added.

See merge request !932

It is now possible to view contributor and commit statistics for
refs beyond the default branch.

Additionally, conditions in RefsController#switch have been refactored
in order that a `case` block is used now.

Use teaspoon instead of jasmine:ci