Create a helper function to create a more detailed error when the
xml parsing fails.

* subversion/libsvn_ra_serf/ra_serf.h
  (svn_ra_serf__xml_parser_t): Remove unneeded flag.

* subversion/libsvn_ra_serf/util.c
  (parse_xml): New function.
  (inject_to_parser,
   svn_ra_serf__process_pending,
   svn_ra_serf__handle_xml_parser): Use parse_xml.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1553907 13f79535-47bb-0310-9956-ffa450edef68