be clearer on type-validation-errors

src/json-validator.cpp

@@ -147,8 +147,8 @@ void validate_type(const json &schema, const std::string &expected_type, const s
 		    (type_instance == "number" && expected_type == "integer"))
 			return;
 
-		throw std::invalid_argument(name + " should be a " + expected_type +
-		                            ", but is a " + type_instance.get<std::string>());
+		throw std::invalid_argument(name + " is " + expected_type +
+		                            ", but required type is " + type_instance.get<std::string>());
 	}
 }