Handle worker failures

[bergie]

We should either set implicit useWorker: false or at least log a better error when worker creation fails.

https://developer.mozilla.org/en/docs/Web/Guide/Performance/Using_web_workers#Handling_errors

[Inviz]

We can do it. Why would worker creation fail besides that its not supported? Something like out of memory?

[bergie]

@Inviz most typical situation I've seen is permissions or wrong worker path