Return spec compliant error objects

JSON-RPC specifies types of error objects. For example, when there isn't a method for the request, the spec requires to response with the following object:
```json
{"jsonrpc":"2.0","error":{"code":-32601,"message":"Method not found"},"id":1}'
```
https://www.jsonrpc.org/specification#error_object

There are already some test cases that address it:
https://github.com/CodeChain-io/jsonrpc-filter/blob/master/test/e2e_test.sh#L72

However, we just don't respond to a such request.
That's why our current test is failing:
https://github.com/CodeChain-io/jsonrpc-filter/commit/cbfb77c1daeeab5fd09399350261f18f53dbd172/checks?check_suite_id=321871236