Define $error page on a folder basis

[stephane-vanraes]

Is your feature request related to a problem? Please describe.
Currently it is only possible to have one $error page (in the root of the project) but in various circumstances it can be nice to have a different error page depending on where you are in the application. (fe: in /products )

Describe the solution you'd like
Have the possibility to define a $error page per folder, in the same way we can define a $layout page per folder.

Describe alternatives you've considered
It is possible to use the page store to differentiate all this logic in the global $error page, just like one would do to differentiate between different status codes.

How important is this feature to you?
The current solution becomes very messy and unmanageable in large applications with custom error pages.

[stephane-vanraes]

possible duplicate of #519 (which somehow didn't turn up in my initial search)