BUG: shift with a negative number should fail

[mattip]

xref gh-13739

A negative shift is undefined in C. Python raises a ValueError, numpy returns the undefined values. We should at least warn, and probably raise an Error.

>>> a = np.arange(4)
>>> 10 << -1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: negative shift count
>>> a << -1
array([-9223372036854775808,                    0, -9223372036854775808,
                          0])

[mattip]

The last example now returns array([0, 0, 0, 0]), but should it raise?