with_parallelizable_workloads should end loop on exception
with_parallelizable_workloads should end loop on exception

Summary: This fixes a bug in `with_parallelizable_workloads`, which could result in the loop not stopping when `f` throws. Now, regardless of whether `f` returns or throws, we always let the current command finish and then stop the loop.

Reviewed By: jbrown215

Differential Revision: D13738839

fbshipit-source-id: 8db2268ed87f80598e09160334b0349970b391ef

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