A hook is added to `container`s of Repository which is called if
`change_head` is attempted but the specified branch does not exist.
This is just to support the previously existing behavior for
`Project`, in which a validation error is added. Technically this
change removes the validation error from Snippet, but that was never a
defined or necessarily desired behavior before. If someone wishes to
add that in the future, it will be easy; they just need to use the
hook in Snippet.