Desired API behaviour for queries with invalid/unknown ecosystems

Currently, the OSV API:
- Returns an error if you query with an `ecosystem` not in the osv-schema
- Returns empty results (no error) if you do a `purl` query with a PURL type that we do not know
- Returns empty results if you query an ecosystem with an unknown (or invaid) suffix (e.g. `Debian:100`, `Alpine:foobar`, `npm:2`)
- (Usually) matches records of any ecosystem suffix even if a suffix is not provided and required (e.g. `Alpaquita` matches `Alpaquita:23`, `Alpaquita:25`, `Alpaquita:stream`, etc)

We should decide and document the behaviour we want, and make sure it's consistent.