[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>

subcmds/start.py

@@ -168,4 +168,6 @@ revision specified in the manifest.
                     % (p.RelPath(local=opt.this_manifest_only), nb),
                     file=sys.stderr,
                 )
+            msg_fmt = "cannot start %d project(s)"
+            self.git_event_log.ErrorEvent(msg_fmt % (len(err)), msg_fmt)
             sys.exit(1)