Hide the informational 'Malformed XML: parsing aborted' errors in ra serf,
when we have a better error message in the error chain.

(See authz_tests.py 25 "remove a subdir with authz file" as an example
 of a simplified error chain)

This resolves a slight regression against 1.8 where we ignored all xml
errors when we had an error in the chain.

* subversion/libsvn_ra_serf/xml.c
  (EXPAT_HAS_STOPPARSER): Remove macro.
  (parse_xml): Update argument to expat context. Handle the inner error here
    instead of in the caller to allow hiding our abort handling.

  (expat_start,
   expat_end,
   expat_cdata): Update abort call.

  (expat_response_handler): Update caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1558745 13f79535-47bb-0310-9956-ffa450edef68