gerrit_util raises GerritAuthenticationError with a nice message
suggesting to check the .netrc file.

R=szager@chromium.org, vadimsh@chromium.org
BUG=369512

Review URL: https://codereview.chromium.org/278203002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@269865 0039d316-1c4b-4281-b951-d872f2087c98