BUG: np.mean does not raise AxisError for illegal axes

Minor, but:
```python
>>> a = np.arange(10)
>>> a.sum(axis=2)
AxisError: axis 2 is out of bounds for array of dimension 1

>>> a.mean(axis=2)
# from _count_reduce_items in methods.py
IndexError: tuple index out of range
```

I'd expect both to raise the same error