Check for document breaks live reload in web worker

[matthewdenobrega]

Line 4 of client/index.js presumes the existence of document, which is not the case in a web worker (which also obviously doesn't have script tags). There should be an additional check here for whether document exists before querying the DOM for script tags.

[SpaceK33z]

See #309 for why it's closed. Also note that PR #298 fixed this.