Avoid dropping errors when handling locking requests over serf.  I
don't know whether a standard server will ever trigger the code path
that drops the error but it's simple for a modified server to return
an error, 412 for example, that gets dropped.

* subversion/libsvn_ra_serf/locks.c
  (determine_error): Check status line if err is not set, check
   reason string is not null.
  (svn_ra_serf__get_lock, svn_ra_serf__lock): Add tracing.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1489526 13f79535-47bb-0310-9956-ffa450edef68