BUG: .loc on MultiIndex with list of indexers including an incomplete slice raises KeyError
Tested on latest version 

``` python
import pandas as pd
midx = pd.MultiIndex.from_product([range(5), ['a', 'b', 'c']])
s = pd.Series(range(15), midx)
s.loc[:, 'a':'c'] # works
s.loc[0:4, 'a':'c'] # works
s.loc[:4, 'a':'c'] # raises KeyError
```

```
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-5-4865919a19c8> in <module>()
----> 1 s.loc[:4, 'a':'c'] # raises KeyError

/Users/shoyer/dev/pandas/pandas/core/indexing.pyc in __getitem__(self, key)
   1125     def __getitem__(self, key):
   1126         if type(key) is tuple:
-> 1127             return self._getitem_tuple(key)
   1128         else:
   1129             return self._getitem_axis(key, axis=0)

/Users/shoyer/dev/pandas/pandas/core/indexing.pyc in _getitem_tuple(self, tup)
    643     def _getitem_tuple(self, tup):
    644         try:
--> 645             return self._getitem_lowerdim(tup)
    646         except IndexingError:
    647             pass

/Users/shoyer/dev/pandas/pandas/core/indexing.pyc in _getitem_lowerdim(self, tup)
    751         # we may have a nested tuples indexer here
    752         if self._is_nested_tuple_indexer(tup):
--> 753             return self._getitem_nested_tuple(tup)
    754
    755         # we maybe be using a tuple to represent multiple dimensions here

/Users/shoyer/dev/pandas/pandas/core/indexing.pyc in _getitem_nested_tuple(self, tup)
    810
    811             # this is a series with a multi-index specified a tuple of selectors
--> 812             return self._getitem_axis(tup, axis=0, validate_iterable=True)
    813
    814         # handle the multi-axis by taking sections and reducing

/Users/shoyer/dev/pandas/pandas/core/indexing.pyc in _getitem_axis(self, key, axis, validate_iterable)
   1267             # nested tuple slicing
   1268             if _is_nested_tuple(key, labels):
-> 1269                 locs = labels.get_locs(key)
   1270                 indexer = [ slice(None) ] * self.ndim
   1271                 indexer[axis] = locs

/Users/shoyer/dev/pandas/pandas/core/index.pyc in get_locs(self, tup)
   3602             elif isinstance(k,slice):
   3603                 # a slice, include BOTH of the labels
-> 3604                 ranges.append(self._get_level_indexer(k,level=i))
   3605             else:
   3606                 # a single label

/Users/shoyer/dev/pandas/pandas/core/index.pyc in _get_level_indexer(self, key, level)
   3527             # otherwise a boolean indexer
   3528
-> 3529             start = level_index.get_loc(key.start)
   3530             stop  = level_index.get_loc(key.stop)
   3531             step = key.step

/Users/shoyer/dev/pandas/pandas/core/index.pyc in get_loc(self, key)
   1167         loc : int if unique index, possibly slice or mask if not
   1168         """
-> 1169         return self._engine.get_loc(_values_from_object(key))
   1170
   1171     def get_value(self, series, key):

/Users/shoyer/dev/pandas/pandas/index.so in pandas.index.IndexEngine.get_loc (pandas/index.c:3651)()

/Users/shoyer/dev/pandas/pandas/index.so in pandas.index.IndexEngine.get_loc (pandas/index.c:3578)()

KeyError: None
```
