Unify CGI error handling
Unify CGI error handling

Use the same error handler for 403s and 502s. This basically ports
the required fixes from error_403 (previous commit) to the 502
error on failed exec(). Two user-visible side effects:

 - error message now says "403 Forbidden" instead of "403"
 - failed exec() gets logged over stderr

(also, use \r\n instead of \n as a line seprator but that has been
fixed up by the parent process before).

muthhus/fcgiwrap,gnosek/fcgiwrap,samboy/fcgiwrap,samboy/fcgiwrap