RoiRequestBatch: Always propagate exceptions back up to the caller when a request fails.
slot.py: ValueSlot: Added stubs for notify_finished, etc.
OpArrayCache: call value_changed signal at the end of execute if any blocks were updated.
(I'm not sure why this is needed, but it was present previously and would have broken when the RequestPool implementation changed.  This commit fixes it.)
Merge branch 'master' into multiprocess-hdf5