[git_trace2] Add logs for critical cmds

Trace logs emitted from repo are not useful on error for many critical
commands. This change adds errors for critical commands to trace logs.

Change-Id: Ideb9358bee31e540bd84a94327a09ff9b0246a77
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/373814
Reviewed-by: Joanna Wang <jojwang@google.com>
Tested-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

git_trace2_event_log.py

@@ -198,9 +198,11 @@ class EventLog(object):
             event["value"] = value
             self._log.append(event)
 
-    def ErrorEvent(self, msg, fmt):
+    def ErrorEvent(self, msg, fmt=None):
         """Append a 'error' event to the current log."""
         error_event = self._CreateEventDict("error")
+        if fmt is None:
+            fmt = msg
         error_event["msg"] = msg
         error_event["fmt"] = fmt
         self._log.append(error_event)