roi.py: Minor cleanup
roiRequestBatch: Fix comment
RequestPool: Free failed/cancelled requests, too.
BlockwiseFileset: When file fails to open, log full traceback.
OpStreamingHdf5Reader: Log timing of reads when debug logging is active.
Request: Propagate exceptions that occurred during completion/failure/cancellation callbacks.
Request: Debug feature: If global_thread_pool has 0 workers, execute all requests synchronously as soon as submit() is called.