//FWIW this doesn't belong in Linux - Security, more like Linux - General
Since you obviously are a stage beyond looking at server generated errorcodes, what kind of errors are you exactly looking for? Are they purely textual, or are they a mix of say per-ID text, layout, styles 'n such? I mean, if that's the case any sniffer won't do and you'll need something more sophisticated unless you want to ogle output all of the day and all of the night. Not that I can suggest any package for that... except
The situation is such that the pages deal mostly with dynamic data so the output of the page can vary greatly between different clients.
actually using different clients?