Desired API behaviour for queries with invalid/unknown ecosystems

[michaelkedar]

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.