Slicing using .loc fails when either boundary falls outside the index
Using version 0.12, is throwing an exception the intended behavior?

```
>>> s = pd.Series(np.arange(10))
>>> s.loc[:15]
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-37-e6f70b3a4e46> in <module>()
----> 1 s.loc[:15]

/Users/sleepingpills/anaconda/envs/research/python.app/Contents/lib/python2.7/site-packages/pandas/core/indexing.pyc in __getitem__(self, key)
    697             return self._getitem_tuple(key)
    698         else:
--> 699             return self._getitem_axis(key, axis=0)
    700 
    701     def _getitem_axis(self, key, axis=0):

/Users/sleepingpills/anaconda/envs/research/python.app/Contents/lib/python2.7/site-packages/pandas/core/indexing.pyc in _getitem_axis(self, key, axis)
    773 
    774         if isinstance(key, slice):
--> 775             self._has_valid_type(key,axis)
    776             return self._get_slice_axis(key, axis=axis)
    777         elif com._is_bool_indexer(key):

/Users/sleepingpills/anaconda/envs/research/python.app/Contents/lib/python2.7/site-packages/pandas/core/indexing.pyc in _has_valid_type(self, key, axis)
    744             if key.stop is not None:
    745                 if key.stop not in ax:
--> 746                     raise KeyError("stop bound [%s] is not in the [%s]" % (key.stop,self.obj._get_axis_name(axis)))
    747 
    748         elif com._is_bool_indexer(key):

KeyError: 'stop bound [15] is not in the [index]'
```

The .ix indexer works as expected here (and returns the items contained in the slice).
