[git-cl] fix bug if user is not logged when fetching tryjob results.

<ExceptionObj>.message worked in py2, but not in py3.

R=ehmaldonado@google.com

Change-Id: I1f1302d453e9142b59de4ccab0298367bffde295
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2631495
Auto-Submit: Andrii Shyshkalov <tandrii@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

git_cl.py

@@ -421,7 +421,7 @@ def _fetch_tryjobs(changelist, buildbucket_host, patchset=None):
   else:
     print('Warning: Some results might be missing because %s' %
           # Get the message on how to login.
-          (auth.LoginRequiredError().message,))
+          (str(auth.LoginRequiredError()),))
     http = httplib2.Http()
   http.force_exception_to_status_code = True