Git worktree's have the ability to be locked in order to spare them from
deletion, e.g. if a worktree is absent due to being located on a
removable disk it is a good idea to lock it. When locking such
worktrees, it is possible to give a locking reason in order to help the
user later on when inspecting status of any such locked trees.
The function `git_worktree_is_locked` serves to read out the locking
status. It currently does not properly report any errors when reading
the reason file, and callers are unexpecting of any negative return
values, too. Fix this by converting callers to expect error codes and
checking the return code of `git_futils_readbuffer`.
