Electrum: `error` property in response is not JSON-RPC 2.0 compliant

Errors returned by the Electrum JSON-RPC API in esplora-electrs are not following the JSON-RPC 2.0 Specification which causes clients integration problems (e.g. https://github.com/checksum0/go-electrum/issues/5).

An error is returned as a string while an [error object](https://www.jsonrpc.org/specification#error_object) is expected.

Sample output for **esplora-electrs**:
```sh
✗ echo '{"jsonrpc": "2.0", "method": "server.version", "id": 0}' | netcat 49.12.127.114 10068
{"jsonrpc":"2.0","result":["ElectrumX 1.16.0","1.4"],"id":0}

✗ 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"}
```

Alternative Electrum server implementations follow the standard. Examples:
**- ElectrumX**
```sh
✗ echo '{"jsonrpc": "2.0", "method": "server.version", "id": 0}' | netcat 49.12.127.114 10068
{"jsonrpc":"2.0","result":["ElectrumX 1.16.0","1.4"],"id":0}

✗ 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**
```sh
✗ echo '{"jsonrpc": "2.0", "method": "server.version", "id": 0}' | netcat 203.132.94.196 51001
{"id":0,"jsonrpc":"2.0","result":["Fulcrum 1.8.2","1.4"]}

✗ 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"}
```