[wptrunner] Log error tracebacks

Forward `exc_info` from stdlib `Logger.exception(...)` [0] to `mozlog`
to help debug the harness. Also, fix a bug revealed by the log (shutdown
event is missing some required fields).

[0]: https://docs.python.org/3/library/logging.html#logging.LogRecord

Bug: None
Change-Id: Id65fa18ec76f2cf9988b8a7113285d35b23bea01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5637754


Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
Reviewed-by: Weizhong Xia <weizhong@google.com>
Cr-Commit-Position: refs/heads/main@{#1316544}