JBoss_Maindeployer: improve feedback against CVE-2010-0738

The exploit against CVE-2010-0738 won't work when using GET or POST.  In the existing code the request would fail and the function would return a nil.  This would be passed to detect_platform without being checked and cause the module to crash ungracefully with the error:

Exploit failed: NoMethodError undefined method `body' for nil:NilClass

The first changes detect a 401 authentication message and provide useful feedback.  Given that if, in any case, 'res' is not a valid or useful response the second change just terminates processing.

I've stayed with the module's coding style for consistency.