Add gsutil.py config hint to 401 GCS error message

This CL adds a hint on how to log into GCS to the 401 error message
displayed to the user.

Bug: 381634957
Change-Id: Id72b5030c018b28a7297b7ff3f279485c6da4d9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6056844
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Alan Cutter <alancutter@chromium.org>

download_from_google_storage.py

@@ -143,7 +143,8 @@ class Gsutil(object):
         if status_code_match:
             return (int(status_code_match.group(1)), out, err)
         if ('ServiceException: 401 Anonymous' in err):
-            return (401, out, err)
+            return (401, out, err + '\nTry running "gsutil.py config" to log '
+                    ' into Google Cloud Storage.')
         if ('You are attempting to access protected data with '
                 'no configured credentials.' in err):
             return (403, out, err)