Check body exists before attaching load listener & rename flag to LOADED_SYNC
Use DOMContentLoaded event, always try to use a worker so error is thrown

DOMContentLoaded event fires sooner fixing the body-async case.
If the user reqeusts a worker, try to use one even if no path is set
so that a helpful error is shown.
Merge pull request #2 from edg2s/script-path-fix

Script path fix