gracefully handle referenced item deletion attempts

[rinusser]

Currently it's possible to delete items with active references, e.g. a food item that's used in an intake entry. Doing do throws an exception in Entity Framework, shutting down the application without actually deleting the item from the database.

Prevent this: add a graceful way of handling the situation:

1. At least catch the exception and show an explanation.
2. Ideally, also disable the "delete" button on items that can't be deleted for any reason.