Add custom HTTP handler for authentication errors

[cyb3r4nt]

This patch gives a possibility to configure custom HTTP handlers, which can change the default authentication error responses in the middlewares.

The default implementation uses http.Error() method, which produces content type text/plain and simple textual messages rendered directly into the response body. In some applications this behavior may not be enough, and there may be a need to produce different responses like JSON or HTML. The AuthErrorHTTPHandler interface allows to intercept authentication errors and write HTTP responses according to specific application requirements.

I also added some additional middleware methods, which allow to configure different AuthErrorHTTPHandler implementations for different HTTP routes. Some routes in the applications may require different response bodies or attributes.