Custom Error Log for Specific URL/File matches

I'd like to be able to provide separate logs for developers who are
devloping in their ~/public_html directories so that any error log
messages for requests made for content that is in one of those
directories would be logged to a file in that user's directory instead
of (or in addition to) the standard error log.

I think I've found sample code that brings me close, but I haven't been
able to figure out how to get the text of the error message. I'd like
to do this for both system-generated errors (i.e., compile errors) as
well as for any STDERR generated by my code.

Thanks. I appreciate the help as always.

Eric
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