Improve error messages for deleting attachments

As a frontend developer I want to have an option to determine that an attachment is still used (by an action for example) once I fail to delete it, so that we can show the user something like:

Error: Deletion failed as the attachment is still in use.

(So in fact we just need a mechanism in the payload of the error response from the server - at least another one then just an error from the postgresql driver in python.)

This maybe also affects device properties that are still in use for device property calibrations.