If user is unauthenticated, svn ls would just hang waiting at
"Password for 'User':", now it'll correctly fail with
"svn: Can't get password".

R=iannucci@chromium.org
BUG=323300,346472

Review URL: https://codereview.chromium.org/178763003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@253004 0039d316-1c4b-4281-b951-d872f2087c98