Use the standard ra serf error reporting from ra serfs unlock handling,
and improve a specific unlock error.

* subversion/libsvn_ra_serf/lock.c
  (svn_ra_serf__unlock): Handle all http status errors instead of just a few.

* subversion/tests/cmdline/lock_tests.py
  (unlock_wrong_token): Use stricter check. Remove TODO.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1551919 13f79535-47bb-0310-9956-ffa450edef68