Vadim's solution works very nicely, btw. I also wanted to log the
user_id to the error (server) log as part of any error message from a
page, but I never did come up with a good way to do that server-wide.
In practice, in the rare ;) cases where a user sees an error on
Production, it's usually possible to manually correlate the error and
access logs to figure out who the user was, if you wish. Having the
user_id in the error message itself (or e.g. in a Notice immediately
after) would be nifty, though.
