Throws error when there is no hot client running

The specific error is:
```
Uncaught ReferenceError: __hotClientOptions__ is not defined
```
The error points to [line 125](https://github.com/G-Rath/webpack-serve-overlay/blob/4719df90c1f3f61246b1ad8b51873e5226cf36d0/errorOverlay.js#L125), which assumes `__hotClientOptions__` exists in the global scope.

Filed this issue at @G-Rath 's request from https://github.com/G-Rath/webpack-serve-overlay/issues/5#issuecomment-411582357 . Due to this error, `webpack-serve-overlay` cannot run on all environments as it implicitly requires a hot client to exist and will error out the bundle otherwise.

Screenie of browser console just in case:

If needed, you can find a reproducible (but not minimal) case in my boilerplate files [here](https://github.com/agilgur5/front-end-base/tree/de36d300006bb70632f3206fd87e9b2253130a4d) by running `npm run build:watch` and opening `index.html`.

Didn't file a PR instead since I'm not sure how you'd want to handle this (there are many options).