Expose some error messages generated by github (or any buggy server) that are transferred as invalid xml.
Expose some error messages generated by github (or any buggy server) that
are transferred as invalid xml.

In the case I encountered the xml document ends with a '\0' byte, which makes
it invalid xml... but the client already received a more specific error
message.

This changes the message I got from
[[
svn: E170013: Unable to connect to a repository at URL 'https://github.com/libgit2/libgit2/tags/v0.23.3'
svn: E175009: The XML response contains invalid XML
svn: E130003: Malformed XML: not well-formed (invalid token)
]]

to

[[
svn: E170013: Unable to connect to a repository at URL 'https://github.com/libgit2/libgit2/tags/v0.23.3'
svn: E175009: The XML response contains invalid XML
svn: E130003: Malformed XML: not well-formed (invalid token)
svn: E200042: Additional errors:
svn: E900002: This repository is hosted on a new server that doesn't support SVN yet. Please contact support if you need SVN access for this repository.
]]

* subversion/libsvn_ra_serf/multistatus.c
  (svn_ra_serf__handle_server_error): If we already recorded an error, return
    that with the xml parser errors of possible further errors.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1707164 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion