Skip 404 errors when looking up Google groups

When checking user membership against Google groups the groups are checked one
at a time and in the order that they were supplied. If one of the groups does
not exist then the checking is halted with the following error.

google.go:201: googleapi: Error 404: Resource Not Found: groupKey, notFound

None of the groups following the missing group are checked either. This means
that something as trivial as a typo in the first group will make it impossible
for anybody to login.

This change catches the 404, logs a message, and then carries on as usual. In
this way a typo will cause a particular group to stop working but will not
affect any other groups.