check_ssh: change warning to critical for protocal/version errors

It makes more sense to exit critical if a explicit version/protocol is requested. This
would also be more consistent with other plugins. Other string matching plugins like
check_snmp or check_http exit critical if the result does not match.

Signed-off-by: Sven Nierlein <sven@nierlein.de>

Closes #1268
Adding a warning for raising to CRITICAL for protocal/version errors of check_ssh