Config test: no error on missing upstream for retrying on HTTP status code

[ironsmile]

Consider the example

                            {
                                "type": "proxy",
                                "settings": {
                                    "try_other_upstream_on_code": {
                                        "404": "does-not-exist"
                                    }
                                }
                            }

If the upstream does-not-exist is true to its name, the nedomi -t will not say the config was wrong. Rather, there will be quite a lot of runtime errors.

[mstoykov]

This will more or less require types.App to be provided to each handler when it's created in New.

This is probably not a bad idea as it will mean that the upstream setting could be a setting of the proxy handler not one of the Location/VirtualHost it's in.

Other places where it's used could benefit from this as well - we can practically remove all the things we put in context currently and get them through the types.App.

The list of other handlers currently using contexts' GetApp and GetContext:

• the status handler
• the purge handler