make mod_proxy flag workers as 'in-error' for all server errors

[Thomas <al...@gmx.net>]

Looking at httpd-2.4.3/modules/proxy/mod_proxy.c (about line 1100) I
noticed there is no error flagging of workers if the access status is an
error and not HTTP_INTERNAL_SERVER_ERROR or HTTP_SERVICE_UNAVAILABLE.
Assume DNS resolution failed, so the reverse proxy cannot resolve the
server's hostname. If this is only temporarily, simply trying again with
the next request might be ok but if it's a permanent situation then it's
simply an error.

I attached a small patch to address this. The code can be simplified a bit
after the patch (not included in the patch for the sake of brevity).

If you have any suggestions to improve the patch, please write me or my
colleague micha@lenk.info.

Cheers