Make svnserve end / close a connection whenever there was an error
that didn't get handled by the respective commands.

This applies to connection failures as well as ordinary SVN errors
and makes threaded execution behave the same as forked mode.

* subversion/svnserve/svnserve.c
  (serve_thread): If there was an error, we are DONE.
                                     Don't try to continue with the next
                                     command ...

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1590985 13f79535-47bb-0310-9956-ffa450edef68