Improve handling for 503 server error

[sallymoc]

When the app tries to fetch data from rpc and the server is not available, it seems the app is still trying to parse the response.

App shouldn't try to parse the response when receiving 503.
App can display a 'user friendly' message indicating the service that has failed and the server not available reason.

[kyubinokitsune-ace]

I can solve this issue.
That error happens because RTK Query’s fetchBaseQuery tries to JSON-parse the response by default. When your RPC is down it returns 503 with an HTML/text body, so RTK Query throws a PARSING_ERROR (it still tries to parse even though the HTTP status is 503).
This is my opinion.
How about you?