In svnserve: don't hide fatal errors in inetd and tunnel modes.

* subversion/svnserve/svnserve.c
  (sub_main): On any fatal error after starting a connection in inetd or
    tunnel mode, report it and exit non-zero instead of exiting silently
    with code zero.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1544256 13f79535-47bb-0310-9956-ffa450edef68