Minor cleanups.
slot: Better formatting of SlotNotReady error message.
MetaDict: Pretty __str__ and __repr__
Revert "Revert "Operator: When an input becomes unready, set all operator outputs to unready, including subslots.""

This reverts commit 0894d6ddef34e4bd39e3dc85f2b0be0dbaf08cc8.
Fixed a bug in the new implementation of Operator.handleInputBecameUnready()
operator.py: Fixed implementation of handleInputBecameUnready
OpMultiArraySlicer2: Removed special behavior when output becomes unready.
That code caused #999.  This fixes it.  The original problem that prompted that code has been fixed in the previous few commits.