Load custom components in error pages

[Error pages](https://github.com/BeaconCMS/beacon/pull/331) do not support custom components at this moment, ie: calling `<%= my_component %>` since it's not safe to do so and requires loading a LV env. For example, a page may break due to a custom component which will trigger an error page, but if that same component is used in the error page it will case an infinite loop. We can revisit this problem when we get to work on https://github.com/BeaconCMS/beacon/issues/84

Start by reading the error-page work in linked pull request 331 and the related issue 84, then trace how error pages render without a LiveView environment. Done means custom components can be used safely in error pages without triggering an infinite error loop.

Written by the indexing model from the issue text.