More helpful panic if you call Work() before you call Ready()

If you set up a worker, but don't call Ready() (which is obviously wrong I'll admit), you get a complex and unhelpful panic because the worker is not in a good state. All this adds is a check that Ready has been called on the worker, if not the code will still panic but with a more helpful message.

I am not sure that a panic is the 100% best solution, but since that is what happens in this situation I thought I'd try and save the next person who hits this issue a safety net.