json handler: Unsupported type serializing oauth error

One of the errors dispatched by the [oauth2 library](https://github.com/golang/oauth2/blob/master/internal/token.go#L256) contains a `http.Response` which causes the following error:

error logging: json: unsupported type: func() (io.ReadCloser, error)

This feels like something `WithError(err)` should handle gracefully, otherwise knowledge of underlying error types needs to be propagated throughout the code.

I'm happy to submit a patch, but not sure what the best solution for this is. Guidance welcome.