trace2_event_log: Added logging of error events.

Added error event in preperation for superproject to log errors.

Testing:
+ Unit tests
   ./run_tests -v

Bug: [google internal] b/189371541
Change-Id: Ife1dd28d52d9e9925b7b34ae913f8eb5fa19037c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/308863
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Xin Li <delphij@google.com>
Tested-by: Raman Tenneti <rtenneti@google.com>