Add message in onAuthenticationError

[Sam Segers]

I had problems trying to see what the error was in my provider using the standard OAuth plugin component.
I used the debugging info of https://developer.ubuntu.com/en/start/platform/guides/online-accounts-developer-guide/ but it only gave a vague error code.

Adding the error message to the output helped me find that the problem I was facing was that the 'state' property isn't passed to the redirection url from my provider using OAuth2.0 (http://tools.ietf.org/html/rfc6749#appendix-A.5).