Fix issue 4044 in ra_serf: return the right error code if the server doesn't
support a particular feature, so that callers of the ra layer can enable a
fallback mechanism or report a meaningful error to the user.

* subversion/libsvn_ra_serf/util.c
  (svn_ra_serf__error_on_status): When the server returns a 501 Not Implemented
      status code, create an SVN_ERR_UNSUPPORTED_FEATURE error.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1511057 13f79535-47bb-0310-9956-ffa450edef68