Fix error message for missing snapshot

[bblommers]

When a key is marked as snapshotted, but not actually present in the snapshot.json, the following error is thrown:

No state for tests/mytest.py recorded. Please (re-)generate the snapshot for this test."

This is very confusing though, as there is a state for tests/mytest.py present - it is just missing the state for a specific key.

This PR updates the error message to make it clear that only the key is missing, instead of the entire snapshot.