hgweb: log error before attempting I/O Previously, an uncaught exception during HTTP request serving would attempt to send an error response then log the exception. If an exception occurred during I/O, this exception would be raised and the original exception wouldn't be logged. This commit changes behavior so the original exception is logged first, before we attempt to do anything else. This ensures the exception is logged. This change resulted in new tracebacks appearing in various tests. Because tracebacks can vary between Python versions, we added a simple script to filter the stack part of traceback lines. This makes testing much simpler, as we don't need to glob over lines and make lines conditional. Differential Revision: https://phab.mercurial-scm.org/D5749