This change introduces a 'p4 login -s' login status check.
If it
fails, the changelist submit attempt is rejected, with an appropriate
message for the user.
Added user message and change submit rejection if the Perforce connection
check fails. This is unlikely as we're called by the Perforce server.
Added user message and change submit rejection if the 'p4 describe' on the
user's changelist fails for whatever reason.