Report error reason during validation

[rmccue]

If an error occurs during validation, the UI displays a useless message with a lack of detail:

Error: Could not parse the authentication response, please forward this error to your administrator: "invalid_response"

This PR handles the common (only?) case, where we have a single error that occurs. It adds the error "reason" to this message:

Error: Could not parse the authentication response, please forward this error to your administrator: "invalid_response" ("SAML Response must contain 1 assertion")

This aids in debugging setup of a SAML integration.