Catch a throw in a fold

[martinsumner]

If something is thrown in a fold, catch and re-throw it. Allow workers to see what's been thrown, without crashing the penciller or inker snapshot (and making sure the penciller or inker snapshot is cleared up).

This is necessary to handle the stop_fold throw used in Riak to close a 2i query when max_results have been reached

[martinsumner]

#184