Seeder superclass warning and error

[lbennett-stacki]

This will warn users about inoperable seeder classes, throwing on the worst case that all seeders are inoperable.

Migrating from the fork origin, I found that I had implements Seeder instead of extends Seeder, which is compilable but invalid. The CLI said it could find the seeders, and that it had executed them. But I had no results. This was because the seeders sent to useSeeders was an empty array.

It might be worth adding a similar check in useSeeders to keep it reusable but up to you 🤷

[lbennett-stacki]

Hey @jorgebodega, thanks for providing a migration path to latest typeorm. This is a small DX improvement I found along the way. Let me know if you want to go in a different direction.

[jorgebodega]

Hi! Those changes are okay, but probably some tests are going to fail.

In any case, you have to update the branch and add some test to cover that new case.