Confused about Trace.axd

[Sally]

Can I set up my web.config and my @page to ONLY produce trace when I
use Trace.Warn?

I have some occasional problems and I want them to get logged, but if
I log EVERY page hit, then I cannot find which ones have errors.

I am trying to get Trace.Warn to do something that it was not intended
for?

Any other ways that I can make a "error log" without writing one
myself?

-SM