[DX] Improve error log when the server isn’t responding

[andreaskienle]

Description

When working with different environments (e.g., a local Go server, canary or other backends), it is possible for the BFF to fail to connect to the intended backend. This can occur if the server is not running, the environment configuration is incorrect, or for similar reasons.

Currently, the error logs in such cases are unclear (server responds with 500), making it hard to quickly diagnose the problem. This can lead to confusion, especially when working locally.

To reproduce:
Run the frontend with env variables configured to target the local Go server, but do not start the server.

We should be more clear about the root cause of the problem maybe using a different error code (502 Bad Gateway or 503 Service Unavailable would be good candidates).