Native JS methods must return JS_FALSE for (Error) exceptions to be thrown

I discovered the first case when FileBase.get_path() failed, but didn't
(immediately) throw an exception. Reviewing the other instances of
JS_RepoertError() calls found several that either reported a garbage (e.g.
NULL) string value or returned JS_TRUE.

The design pattern used a little in js_socket.c probably should be used more:

	if (JS_IsExceptionPending(cx))
		return JS_FALSE;
	return JS_TRUE;

... but that's more of a refactor than I had the stomach for right now.