Deleting Collections buggy

[michaelbromley]

Describe the bug
2 things:

1. The toast messages refer to the collection as a "ProductVariant" (copy paste error 🙃)
2. Attempting to delete a parent collection will cause a DB-level foreign key violation error. This should actually be checked for in the service layer and respond with a more helpful error.

Environment (please complete the following information):

• @vendure/core version: 0.4.0
• Database (mysql/postgres etc): mysql