Add option to fail hard if there should be valid top level signature and validation of top level signature failed

[cjbarth]

Currently, if the validation of the top-level signature fails, we continue after setting a variable. We should have an option, or perhaps just fail, if we find a top-level signature that isn't valid. Right now, node-saml will allow an invalid top-level signature. This allowance allows for a whole class of crafty attacks that cause a top-level signature to be invalid, but which preserve the signature of an assertion. See node-saml/passport-saml#671 for a more thorough discussion of this matter.