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>

git_trace2_event_log.py

@@ -159,6 +159,13 @@ class EventLog(object):
       def_param_event['value'] = value
       self._log.append(def_param_event)
 
+  def ErrorEvent(self, msg, fmt):
+    """Append a 'error' event to the current log."""
+    error_event = self._CreateEventDict('error')
+    error_event['msg'] = msg
+    error_event['fmt'] = fmt
+    self._log.append(error_event)
+
   def _GetEventTargetPath(self):
     """Get the 'trace2.eventtarget' path from git configuration.