Add a CI check that fails if we are missing a test for a user error

### What is wrong?

Some of the errors that the compiler is supposed to present to the user aren't covered by tests.

### How can it be fixed

Add a CI check that:

1. extracts a unique part of each user facing error message that the compiler produces
2. Check that the unique part is somewhere present in our snapshots

That should ensure we do not end up in a situation where we have uncovered compiler errors.

Start by locating where the compiler's user-facing errors are defined or emitted, where their snapshots are stored, and how CI checks are configured. Determine how to extract a unique message part and compare it with the snapshots; done means CI fails when an error has no matching snapshot and passes for covered errors.

Written by the indexing model from the issue text.