More validation of OAuth error responses
More validation of OAuth error responses

It's possible a server may respond a response in JSON with the key
"error" that does not conform to Section 5.2 of RFC 6749.
https://tools.ietf.org/html/rfc6749#section-5.2

WilliamDenniss/AppAuth-iOS,WilliamDenniss/AppAuth-iOS,WilliamDenniss/AppAuth-iOS,openid/AppAuth-iOS,openid/AppAuth-iOS,openid/AppAuth-iOS