Better propagation of underlying errors

[mitar]

In client_authentication.go there is code like:

			client, err = f.Store.GetClient(ctx, clientID)
			if err != nil {
				return nil, errors.WithStack(ErrInvalidClient.WithDebug(err.Error()))
			}

This means that all the information about the err (stack trace, other wrapped errors) is lost at that point. Could RFC6749Error implement WithError and Unwrap to allow chaining errors? So then the above would be:

return nil, errors.WithStack(ErrInvalidClient.WithError(err).WithDebug(err.Error()))

External facing nothing would change, but if you inspect returned error, you would have access to the underlying wrapped error using (now) standard error wrapping.

[aeneasr]

100% on this one!

[mitar]

Awesome.

[mitar]

I made: #479