Return 404 on unknown queries

We should only return an empty response when we recognize all parts of a query.

These queries should return a `404` with an error message:
- `/api/ballots/?election_id=999999`
- `/api/ballots/?precinct_county="Not A Real Place"`

Start by tracing the /api/ballots/ entry point and how election_id and precinct_county filters are handled. Confirm the current behavior for the two example queries, then make unknown election or location values return a 404 with an error message while recognized empty results remain unchanged.

Written by the indexing model from the issue text.