Fix #4378 - Do exception handling

Fix #4378
Take full advantage of the check command
This oracle scheduler exploit hangs if not vuln

When this exploit gets run against a system that isn't vulnerable
it can hang for a signifigant ammount of time. This change uses the check
method on the exploit to see whether it should proceed. Don't try to exploit
the host if it's not vulnerable.
Land #4404, Oracle scheduler exploit fix
Land #4402, ms12_020_check NilClass fix