fix: check response before unmarshaling

[lesterli]

Summary

We saw the following unmarshalling error in the FG logs.

2024-11-24T01:27:45.282737Z	info	Processing new blocks	{"start_height": 138620, "end_height": 138622}
2024-11-24T01:27:45.282774Z	info	Processing batch of blocks	{"batch_start_height": 138620, "batch_end_height": 138622}
2024-11-24T01:27:46.315131Z	error	Error checking if block is finalized from babylon	{"block_height": 138622, "error": "error in json rpc client, with http response metadata: (Status: 502 Bad Gateway, Protocol HTTP/1.1). error unmarshalling: invalid character 'e' looking for beginning of value"}
2024-11-24T01:27:46.315187Z	fatal	Error processing blocks	{"error": "error processing block 138622: error checking is block 138622 finalized from babylon: error in json rpc client, with http response metadata: (Status: 502 Bad Gateway, Protocol HTTP/1.1). error unmarshalling: invalid character 'e' looking for beginning of value"}

Upon checking the code, the error is thrown in the QueryIsBlockBabylonFinalizedFromBabylon function, and only cw-related functions involve JSON unmarshalling, this PR checks the response before unmarshaling it.

Test Plan

make lint
make test