Fix smb_login calling nonexistent method

When a Rex::Proto::SMB::Exceptions::InvalidWordCount exception is thrown by this module, it attempts to call the nonexistent method error_reason and throws a NoMethodError:

Auxiliary failed: NoMethodError undefined method `error_reason' for #<Rex::Proto::SMB::Exceptions::InvalidWordCount:0x007f48fcda0e48>

This changes uses the built in method get_error to return an error code.

[-] x.x.x.x:445 SMB - [1/1] - \\Domain - FAILED LOGIN (xxxxxxxx) xxxx : xxxxx [STATUS_WAIT_0]