Allow custom error handlers via hooks

[florianmrz]

I recently wanted to add a custom error handler to our Kirby, but there's no easy way to do so. In theory it is very simple, because Kirby uses Whoops which allows you to register multiple handlers for different errors. To add my custom logging logic to the existing ones from Kirby, I had to overwrite the logic for all three error handlers in \Kirby\Cms\AppErrors.

It would be a lot easier if Kirby allowed you to register custom error handlers for all three error types (CLI, HTML and JSON), possibly via the already existing hooks.

[bastianallgeier]

Ohhhh, that's a lovely idea and indeed not very difficult! This would also solve the wish of many to customize the fatal error screen.