009 attention 500s on bad input: a Flow PT REST behaviour that should change

Expected A request naming a row that is not there is a 404, and a request naming an entity type the server does not accept is a 400. The site's own /spec.json gives activity_stream a 404 for a record that is not there.

Actual

call	answer
`GET /entity/shots/999999999/activity_stream`	`500 Shotgun Server Error`
`GET /entity/bogus_things/1/activity_stream`	`404 Entity type 'bogus_things' does not exist.`
`POST .../follow` with `{"entities": [{"record_id": N, "entity": "shots"}]}`	`500 Shotgun Server Error`
`POST .../follow` with `{"entities": [{"record_id": N, "entity": "Bogus"}]}`	`400 {"entities": {"0": {"entity": ["entity is not valid"]}}}`
`POST .../follow` with `{"entities": [{"record_id": N, "entity": "Shot"}]}`	`204`, followed

An unknown entity type is handled on both calls. A missing row and a plural entity name are not.

shots is the spelling every URL on this API uses, /entity/shots/<id>, so the value that 500s is the one a caller reaches for first, and an outright invalid name gets the better error.

Reproduce

curl -sS -o /dev/null -w '%{http_code}\n' \
  "$SITE/api/v1/entity/shots/999999999/activity_stream" -H "Authorization: Bearer $TOKEN"
# 500

curl -sS -X POST "$SITE/api/v1/entity/human_users/<user_id>/follow" \
  -H "Authorization: Bearer $TOKEN" -H "Content-Type: application/json" \
  -d '{"entities":[{"record_id":<id>,"entity":"shots"}]}'
# 500

curl -sS -X POST "$SITE/api/v1/entity/human_users/<user_id>/follow" \
  -H "Authorization: Bearer $TOKEN" -H "Content-Type: application/json" \
  -d '{"entities":[{"record_id":<id>,"entity":"Bogus"}]}'
# 400, which is the answer the plural should get

Impact A 500 is retried by every client that retries anything, so a stale record id becomes repeated load on the server rather than an error the caller handles. It also tells the caller nothing, so the plural case reads as an outage rather than a bad request, and the difference between shots and Shot is not something the response can teach.

Proposed change Answer 404 for a record id that does not exist, matching the site's own spec, and 400 for an entity value the endpoint does not accept, matching what an unknown name already gets on the same call.