Throw an error instead of a string

Sorry, saw this yesterday and meant to PR earlier.

Per http://www.nczonline.net/blog/2009/03/10/the-art-of-throwing-javascript-errors-part-2/ (which is a few years out of date, but still)

> ```js
throw "message";
```
> _Doing so will cause an error to be thrown, but not all browsers respond the way you’d expect. Firefox, Opera, and Chrome each display an “uncaught exception” message and then include the message string. Safari and Internet Explorer simply throw an “uncaught exception” error and don’t provide the message string at all. Clearly, this is suboptimal from a debugging point of view._

Although if this is pure server-side only, perhaps not an issue (depending on how far up the error catches).