REF: simpilify _cython_agg_general
- make the exception-handling in SeriesGroupBy._cython_agg_general match that in DataFrameGroupBy._cython_agg_general
- remove an unnecessary for loop
- define array_func to match the pattern in DataFrameGroupBy._cython_agg_general in the hopes that we can share these methods before long
great, keep up the de-tangling!