[Improvement] Gravitino server should not return 404 when entity like metalake, user not found

### What would you like to be improved?

When we call `http://localhost:8090/ui/metalakes?metalake=test1` and metalake `test1` does not exist, front page return 404

The response code 404 will confuse users.

### How should we improve?

_No response_

Start by tracing the server entry point for /ui/metalakes?metalake and how a missing metalake is handled. Check the existing endpoint tests, if present, and clarify the intended response before changing behavior; done means a nonexistent entity no longer produces a confusing front-page 404.

Written by the indexing model from the issue text.