Improve error message for missing gerrit credentials in git-cl.

Bug:
Change-Id: I058fcd85548c6e17f33de71ed2ad04549294e13d
R: dpranke@chromium.org, iannucci@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/558250
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

gerrit_util.py

@@ -140,6 +140,10 @@ class CookiesAuthenticator(Authenticator):
 
   @classmethod
   def get_new_password_message(cls, host):
+    if host is None:
+      return ('Git host for gerrit upload is unknown. Check your remote '
+              'and the branch your branch is tracking. This tool assumes '
+              'that you are using a git server at *.googlesource.com.')
     assert not host.startswith('http')
     # Assume *.googlesource.com pattern.
     parts = host.split('.')