v8: add type check to make failed calls visible

Calling v8.setFlagsFromString with e.g a function as a flag argument gave no exception or warning that the function call will fail. There is now an exception if the function gets called with the wrong flag type (string is required) or that a flag is expected. v8 flags are only possible as strings. Other APIs (fs or net) already provide exceptions if the argument has not the expected type.