Copy/Move of non existing resources must not return a 204

Currently using `/copy` or `/move` with a non existing page da-admin does nothing but return a 204 status. If the source is not existing da-admin should return a 404.

Start by locating the `/copy` and `/move` request handlers and reproduce each operation with a nonexistent source page. Trace the current response path, then verify that a missing source returns 404 while valid copy and move operations retain their existing behavior.

Written by the indexing model from the issue text.