Make mod_dav_svn properly report 'svn unlock' errors by producing direct output
instead of waiting for mod_dav to forget handling any error messages and just
reporting a server error on any error.

* subversion/mod_dav_svn/lock.c
  (remove_lock): Return NULL instead of 0.
  (remove_lock_svn_output): New function, wrapping remove_lock.
  (dav_svn__hooks_locks): Use remove_lock_svn_output.

* subversion/tests/cmdline/lock_tests.py
  (block_unlock_if_pre_unlock_hook_fails): Require explicit output instead of
    accepting a HTTP error.
  (lock_hook_messages): New function.
  (test_list): Add lock_hook_messages.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1553701 13f79535-47bb-0310-9956-ffa450edef68