We occasionally observe wrong response code 401 sent by Gerrit (current
stable-2.16 version) when authentication of REST request fails since
LDAP was shortly not available.
If Gerrit cannot authenticate the request since the authentication
backend wasn't reachable it should respond with 503 to signal the client
it should retry later.
Issue: Issue 12786
Change-Id: I159fa98ab7f2b3c8166648ff90da2554aeadd276