ccsr: Fix error code for subject not found

[def-]

See https://docs.confluent.io/platform/current/schema-registry/develop/api.html#errors for error codes

Caused by #27641

Nikhil noted this correctly in the original review: #27641 (comment)

[github-actions]

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[teskje]

Are you sure "404" can never happen? In the linked docs I see some APIs that document "404 Not Found" but no sub-error codes.

[def-]

Only if the json body can't be parsed would we return the 404 error code, and would then wrongly consider it as a "SubjectNotFound". All the other error handlers in this file do it the same way.

[teskje]

For example, the GET /config/{subject} API documents these errors:

Status Codes:
 * 404 Not Found – Subject not found
 * 500 Internal Server Error –
      Error code 50001 – Error in the backend datastore

What error code does the API return for "Subject not found", assuming the response can be parsed? It looks like it's 404. Are you suggesting there is a bug in the documentation or am I missing something?

Btw, for PublishError we also match on "409", not a five-digit error code. Is that also a bug?

[def-]

From what I can tell their documentation is kind of misleading. The code seems to always set the error code to 40401 for subject not found: https://github.com/confluentinc/schema-registry/blob/master/core/src/main/java/io/confluent/kafka/schemaregistry/rest/exceptions/Errors.java#L25

The 409 is also a bug, fixing.

[teskje]

Thanks!