python: negative slicing with tail

```Python
In [1]: from blaze import *

In [2]: s = Symbol('s', 'var * int')

In [3]: compute(s[-1], [1, 2, 3])
Out[3]: 3

In [4]: compute(s[-1], iter([1, 2, 3])) # works on lazy sequences too
Out[4]: 3
```
we probably can't support that easily so i think we should raise

```
In [14]: islice(x, -2, -10, -1)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-14-70c44626f9ea> in <module>()
----> 1 islice(x, -2, -10, -1)

ValueError: Indices for islice() must be None or an integer: 0 <= x <= maxint.
```