BUG: Fix TypeError in _cython_agg_blocks
and change one of the high-level `except Exception`s in groupby.groupby to `except (TypeError, NotImplementedError)`.  The remaining `TypeError` will be addressed in a separate PR.  Haven't decided about the remaining NotImplementedError.

cc @jreback @WillAyd.
> As mentioned in other PR I am hoping to do away with this block management entirely so not sure how much you want to invest in this...

I still very much want to see that happen.  FWIW this should be the last PR in this sequence directly touching cython_agg_blocks.  After this there is one more PR that implements group_add for complex dtypes at which point we can stop catching the `(TypeError, NotImplementedError)` in groupby.py:L1360. thanks @jbrockmendel 