Handle error string returned in the JSON-RPC response

[nkuba]

Closes: #5

Here we provide a workaround for servers that don't follow the JSON-RPC 2.0 specification for error objects.

According to the specification, an error should be an object containing code, message, and data properties (see: https://www.jsonrpc.org/specification#error_object).
Unfortunately, Electrs returns an error as a string (see: Blockstream/esplora#453).

We define an error unmarshaling function to handle both types of errors.

Sample outputs from servers that the client has to handle:
ElectrumX

✗ echo '{"jsonrpc": "2.0", "method": "blockchain.block.header", "params": [4294967295], "id": 0}' | netcat 49.12.127.114 10068
{"jsonrpc":"2.0","error":{"code":1,"message":"height 4,294,967,295 out of range"},"id":0}

Fulcrum

✗ echo '{"jsonrpc": "2.0", "method": "blockchain.block.header", "params": [4294967295], "id": 0}' | netcat 203.132.94.196 51001
{"error":{"code":1,"message":"Invalid height"},"id":0,"jsonrpc":"2.0"}

Esplora/Electrs

✗ echo '{"jsonrpc": "2.0", "method": "blockchain.block.header", "params": [4294967295], "id": 0}' | netcat 35.225.54.191 50001
{"error":"missing header","id":0,"jsonrpc":"2.0"}

[losh11]

Can this PR please be merged?