Route and Resource Loading: Support error pages

[leandrocp]

Phoenix Endpoint renders templates for errors given the Endpoint :render_errors is properly set, and Beacon integrates with this mechanism by raising custom exceptions with a proper :plug_status.

Currently the only supported way to render these error page is by defining them in the host application, but we want to allow users to write the templates for 4xx, 5xx errors at runtime.

Tasks

Beacon

• Add table beacon_error_pages
• Add schema ErrorPage with at least :template and :status fields. Status must be one of the 4xx and 5xx as listed at https://github.com/elixir-plug/plug/blob/main/lib/plug/conn/status.ex#L8
• Functions in Beacon.Content to manage these error pages. A good example is Implement PageEventHandlers #328
• Change the install script to modify the Endpoint :render_errors config value to html: BeaconWeb.ErrorHTML
• Compile a "error module" per site containing all these error templates in Beacon.Loader. Similar to how Layouts are loaded.
• In BeaconWeb.ErrorHTML is must pattern match some private assign to find the source site and call the correct render function from the error module for that site. This module acts as a proxy. That's the part we're unsure and should do some research.

Beacon LiveAdmin

• Add a new menu item "Error Pages" where users can create a template for each available status. A good example is Page Events beacon_live_admin#50