- [JSONRPC] improved error message.

src/json-rpc2.lisp

@@ -364,8 +364,11 @@
          (error 'json-rpc-error
                 :code (response-error-code +error-invalid-params+)
                 :transaction-id (id request)
-                :text (format nil
-                              "Number of parameters (arity) not compatible with function")))
+                :text
+                (format nil
+                        "Number of parameters (arity) not compatible with function: expected ~a got ~a."
+                        (length (params request))
+                        (length (params fun)))))
         (t
          (let* ((params           (params request))
                 (called-by-name-p (called-by-name-p fun params)))