loc allows different length boolean indexer 
Is this a bug, or deliberate?

```python
In [45]: a.loc[[True, False]]  # too few?
Out[45]:
a    1
dtype: int64

In [46]: a.loc[[True, False, True, False, False]]  # too many?
Out[46]:
a    1
c    3
dtype: int64
```

The fact that we raise here, when we have too many true values makes me think this is a bug

```python
In [53]: a.loc[[True, False, True, False, True]]
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
~/Envs/dask-dev/lib/python3.7/site-packages/pandas/core/indexing.py in _getbool_axis(self, key, axis)
   1517         try:
-> 1518             return self.obj._take(inds, axis=axis)
   1519         except Exception as detail:

~/Envs/dask-dev/lib/python3.7/site-packages/pandas/core/series.py in _take(self, indices, axis, is_copy)
   3925         indices = ensure_platform_int(indices)
-> 3926         new_index = self.index.take(indices)
   3927

~/Envs/dask-dev/lib/python3.7/site-packages/pandas/core/indexes/base.py in take(self, indices, axis, allow_fill, fill_value, **kwargs)
    793                                                fill_value=fill_value,
--> 794                                                na_value=self._na_value)
    795         else:

~/Envs/dask-dev/lib/python3.7/site-packages/pandas/core/indexes/base.py in _assert_take_fillable(self, values, indices, allow_fill, fill_value, na_value)
    819         else:
--> 820             taken = values.take(indices)
    821         return taken

IndexError: index 4 is out of bounds for axis 0 with size 3

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
<ipython-input-53-9fd96bbd72ce> in <module>
----> 1 a.loc[[True, False, True, False, True]]

~/Envs/dask-dev/lib/python3.7/site-packages/pandas/core/indexing.py in __getitem__(self, key)
   1498
   1499             maybe_callable = com.apply_if_callable(key, self.obj)
-> 1500             return self._getitem_axis(maybe_callable, axis=axis)
   1501
   1502     def _is_scalar_access(self, key):

~/Envs/dask-dev/lib/python3.7/site-packages/pandas/core/indexing.py in _getitem_axis(self, key, axis)
   1867             return self._get_slice_axis(key, axis=axis)
   1868         elif com.is_bool_indexer(key):
-> 1869             return self._getbool_axis(key, axis=axis)
   1870         elif is_list_like_indexer(key):
   1871

~/Envs/dask-dev/lib/python3.7/site-packages/pandas/core/indexing.py in _getbool_axis(self, key, axis)
   1518             return self.obj._take(inds, axis=axis)
   1519         except Exception as detail:
-> 1520             raise self._exception(detail)
   1521
   1522     def _get_slice_axis(self, slice_obj, axis=None):

KeyError: IndexError('index 4 is out of bounds for axis 0 with size 3')
```

In the docs at http://pandas-docs.github.io/pandas-docs-travis/user_guide/indexing.html#boolean-indexing we say

> You may select rows from a DataFrame using a boolean vector the same length as the DataFrame’s index

Note that this only affects `.loc`. `Series.__getitem__` and `DataFrame.__getitem__` are unaffected.