ra_serf: Improve SSL certificate failure message.

* subversion/libsvn_ra_serf/util.c
  (ssl_server_cert): Provide detailed error message about SSL certificate 
   verification.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1514785 13f79535-47bb-0310-9956-ffa450edef68