Add a new test for multi-path lock/unlock callback returning an error.

* subversion/tests/libsvn_fs/locks-test.c
  (struct lock_many_baton_t): Add count.
  (lock_many_cb): Return error.
  (lock_cb_error): New test.
  (test_funcs): Add new test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1605826 13f79535-47bb-0310-9956-ffa450edef68