Improve JSON-RPC error messages and logging

- Pre-initialize method = "" instead of locals().get() in the
  internal-error log path
- Reject JSON arrays with a clear "Batch requests not supported"
  message rather than the generic "Request must be object"

Author: ping-ke

quarkchain/cluster/jsonrpc_server.py

@@ -92,8 +92,11 @@ def wrapper(f):
     async def dispatch(self, request_json: Dict[str, Any], context=None) -> Optional[Dict[str, Any]]:
         req_id = None
         is_notification = False
+        method = ""
 
         try:
+            if isinstance(request_json, list):
+                raise InvalidRequest("Batch requests not supported")
             if not isinstance(request_json, dict):
                 raise InvalidRequest("Request must be object")
 
@@ -147,7 +150,7 @@ async def dispatch(self, request_json: Dict[str, Any], context=None) -> Optional
                 "id": req_id,
             }
         except Exception:
-            logger.exception("Internal JSON-RPC error for method %s", locals().get("method", "<unknown>"))
+            logger.exception("Internal JSON-RPC error for method %s", method)
             if is_notification:
                 return None
             return {