Trace Tests: Update retracing script. Adds an option to skip retracing traces that already exist. That makes the script much easier to use on folders that already have traces partially retraced. It also catches exceptions when running the traces so that ANGLE crashes and ASSERTions don't need you to restart the retrace. Bug: angleproject:5133 Change-Id: I059ca545145eadc1aa8c373a2fde762bbeb52b55 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3018302 Reviewed-by: Cody Northrop <cnorthrop@google.com> Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>