BUG: Series[int] + datetime64 should raise

[jbrockmendel]

identified here. We handle this correctly for timedelta64, but apparently not datetime64

iser = pd.Series(range(3))
dt = np.datetime64(3, "D")

>>> iser + dt
0   1970-01-04
1   1970-01-05
2   1970-01-06

Should behave like iser + pd.Timestamp(dt):

>>> iser + pd.Timestamp(dt)
[...]
ValueError: Cannot add integral value to Timestamp without freq.