Add additional GIDSignInErrorCode when access to resources is denied

[Vaidios]

Hey

Currently, when access to resources is denied, GoogleSignIn return "kGIDSignInErrorCodeUnknown" with description "access_denied". Can we create an additional case called "kGIDSignInErrorCodeAccessDenied", which would be mapped from "OIDErrorCodeOAuthAccessDenied"?

In my case it would help a lot, as we send analytics events when user doesn't authorise access to resources.

[petea]

@Vaidios thanks for the suggestion. We'll take a look at this for the next major release as it would be a breaking change.