superoject: log error message in the 'fmt' field also.

Tested:
+ Verified error messages are being collected.

Bug: [google internal] b/193711236
Change-Id: I6c608a2af332ccd38722b7f83a82e5ac8fa143db
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/317162
Reviewed-by: Xin Li <delphij@google.com>
Tested-by: Raman Tenneti <rtenneti@google.com>

git_superproject.py

@@ -128,7 +128,7 @@ class Superproject(object):
     """Logs message to stderr and _git_event_log."""
     if self._print_messages:
       print(message, file=sys.stderr)
-    self._git_event_log.ErrorEvent(message, '')
+    self._git_event_log.ErrorEvent(message, f'{message}')
 
   def _LogError(self, message):
     """Logs error message to stderr and _git_event_log."""