BUG: dont' coerce reductions in a groupby always to datetimes; only when we have
     actual Timestamps in the data (GH5788,GH5789)
INT: allow internal errors in block construction to bubble up
BUG: resolved GH5788 under numpy < 1.7 because vstack is odd with M8[ns]
BUG: remove convert_numeric=True from groupbys
     perform soft-conversion of numeric dtypes instead
Merge pull request #5790 from jreback/apply_bugs

BUG: dont' always coerce reductions in a groupby always to datetimes