Surround the complete python worker with the try block

This is for making sure depickling and any other nasty errors are captured when they arrive before the function is actually being executed

@Shopify/data-engineers 

An example of this error occuring without a meaningful stacktrace being returned can be found here: https://gist.github.com/bouk/aee201b813b395c23733

I can't test this out because that would require setting up my own cluster or something idk