Fix EZP-23046: kernel error overrides only work once

https://jira.ez.no/browse/EZP-23046

Kernel errors on content view wouldn't set the `error_type` and `error_number`design keys required by overrides on `error/view`.

This adds those keys to the module result of `error/view`, and makes sure that the cache retrieval method adds those to the design keys when applicable.