[auth] Clarify what account to use

There's some confusion on what account to use to login.  Add the
context for Gerrit to hopefully make it a bit easier to decide.

Bug: b/342261857
Change-Id: Ie59b156c2a91f8168bcd2f381f85fb8157040d80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6178581
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Allen Li <ayatane@chromium.org>

auth.py

@@ -68,8 +68,9 @@ class GitLoginRequiredError(Exception):
     """
 
     def __init__(self):
-        msg = ('You are not logged in. Please login first by running:\n'
-               '  %s' % self.login_command)
+        msg = (
+            'You are not logged in to Gerrit. Please login first by running:\n'
+            '  %s' % self.login_command)
         super(GitLoginRequiredError, self).__init__(msg)
 
     @property