fix: update error handling in stream posts handlers

[MiguelMedeiros]

When I received a stream of posts that exceeded the limit, I encountered a 500 Internal Server Error.
I have now updated it to a more appropriate error message, a 404 Not Found error.

Pre-submission Checklist

For tests to work you need a working neo4j instance with the example dataset in docker/db-migration

• Testing: Implement and pass new tests for all new code, while maintaining existing test suite, cargo test.
• Performance: Ensure new code has relevant performance benchmarks, cargo bench

[SHAcollision]

Good catch! 🚀 Not sure why we were using InternalServerError for a empty list of posts. Looks like a typo/bug. LGTM!