Add custom error page for unavailable upstream

[dennissetiawan]

Is your feature request related to a problem? Please describe.

This is related to envoy issues that previously blocked this. (envoyproxy/envoy#1178) But the issue has been resolved recently.

Describe the solution you'd like

I need to have a custom page for upstreams service that returns unavailable (503) error code.
So in case of unavailable service we can choose to :

1. provide our own custom page that we create
2. use pomerium default error page template page that is more user friendly
3. Do redirection globally

Additional Context
It's great if we can configure the solutions at route level also

[kenjenkins]

Hi @dennissetiawan, as a first step we'll change the default behavior to return an HTML error page — basically option (2) as you've listed above.

[abdennour]

What's news here ? shall we use decorators ?