Help the administrator to update custom reports after migration

The migration script should emit warnings about deleted models and fields used by custom reports, as the errors resulting from that when trying to execute a report with missing fields are all but intuitively intelligible.

This could also be a byproduct of https://blueprints.launchpad.net/openupgrade-server/+spec/service-module-feature (it actually should because the orm is not fully loaded when the migration script runs)