Raise exception for bad arguments on batched background migration helper

What does this MR do and why?

Raises an exception when the arguments are incorrect for ensure_batched_background_migration_is_finished. If this fails and is not caught during review, there will be no warning from the pipeline that there has been a failure.

Update: I have changed my approach a bit. From various iterations, it appears that there are a lot of tests and places in the code itself where the "warn silently" functionality is relied upon, and it would take a fair amount of refactoring to fix every spot where it's found.

Instead, we simply look for DBLAB_ENVIRONMENT to be set, which indicates we're running in a db-testing pipeline. I have demonstrated that a purpose-crafted migration fails in this MR: !113898 (closed).

Related to #374924 (closed)