Error Handling Improvements

[josephdecock]

• Make ProtocolResponse.HttpResponse nullable (these null values already happen in error cases, so this just makes the type reflect what can actually happen) and check for null HttpResponses as necessary (fixes NullReferenceException in ProtocolResponse.FromHttpResonseAsync #532)
• Improved the error message in the case where discovery succeeds, but jwks fails
• Don't suppress errors when reading protocol response content (fixes Please add some log or re-throw in case of exception #507)
• Improve authority validation error message (fixes Potential over validation of OIDC URI #553)