Show a more specific error message when an error occurs during the processing of a SAML AuthnRequest

[phavekes]

This issue is imported from pivotal - Originaly created at Jun 10, 2016 by Pieter van der Meulen

When there is en error during the verification of an AuthnRequest a generic "Onherstelbare fout" / "Fatal error" message is shown. The details of the error are logged. To help SPs diagnose some common cases themselves we want to distinguish between different kind of errors.

• The Issuer in the AuthnRequest is not know (i.e. the EnityID of the SP is not configured on the Gateway)
• An unsupported signature algorithm is used, or the request is unsigned
• There was another error during the validation of the AuthnRequest (e.g. signature verification)

Showing the "generic error screen" with time, ART etc would be sufficient.

[phavekes]

(Pieter van der Meulen - Jun 10, 2016)