Make exception handling identical to before the switch to SAML2
Make exception handling identical to before the switch to SAML2

In the previous situation, response processing threw the following
exceptions:

 - No success response: sspmod_saml_Error
   -> received error status code error page

 - Decryption errors, no assertions or precondition errors: SimpleSAML_Error_Exception
 - Signature format or assertion validation errors: Exception
   -> generic verification error page

 - All other errors -> response processing failed error page

This commit makes the new situation act identical:

 - Catch PreconditionNotMetException, see if it was about an SAML
   error response and if so, show the 'received error status code'
   error page

 - Show the response processing error page for all other
   PreconditionNotMetExceptions

 - Show the verification error pages for all other exceptions

thijskh/OpenConext-engineblock,thijskh/OpenConext-engineblock,thijskh/OpenConext-engineblock,thijskh/OpenConext-engineblock