Aligning on multi-index with swapped levels gives unclear error message
When trying to do a calculation with two dataframes with both multi-indexed rows, but with swapped levels, you get the error message `"NotImplementedError: merging with more than one level overlap on a multi-index is not implemented"`. 

This is not very informative in this case.

```
In [11]: df = pd.DataFrame({'a':np.random.randn(6)}, index=pd.MultiIndex.from_product([['a', 'b'],[0,1,2]], names=['levA', 'levB']))

In [12]: df2 = df.copy()

In [13]: df2.index = df2.index.swaplevel(0,1)

In [14]: df
Out[14]:
                  a
levA levB
a    0    -0.248626
     1     2.382662
     2     1.349179
b    0     0.720629
     1     0.205297
     2     0.113340

In [15]: df2
Out[15]:
                  a
levB levA
0    a    -0.248626
1    a     2.382662
2    a     1.349179
0    b     0.720629
1    b     0.205297
2    b     0.113340

In [16]:

In [16]: df - df2
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-16-ffb739d7f3d7> in <module>()
----> 1 df - df2

c:\users\vdbosscj\scipy\pandas-joris\pandas\core\ops.pyc in f(self, other, axis,
 level, fill_value)
    813     def f(self, other, axis=default_axis, level=None, fill_value=None):
    814         if isinstance(other, pd.DataFrame):    # Another DataFrame
--> 815             return self._combine_frame(other, na_op, fill_value, level)
    816         elif isinstance(other, pd.Series):
    817             return self._combine_series(other, na_op, fill_value, axis,
level)

c:\users\vdbosscj\scipy\pandas-joris\pandas\core\frame.pyc in _combine_frame(sel
f, other, func, fill_value, level)
   3099
   3100     def _combine_frame(self, other, func, fill_value=None, level=None):
-> 3101         this, other = self.align(other, join='outer', level=level, copy=
False)
   3102         new_index, new_columns = this.index, this.columns
   3103

c:\users\vdbosscj\scipy\pandas-joris\pandas\core\generic.pyc in align(self, othe
r, join, axis, level, copy, fill_value, method, limit, fill_axis)
   3146                                      copy=copy, fill_value=fill_value,
   3147                                      method=method, limit=limit,
-> 3148                                      fill_axis=fill_axis)
   3149         elif isinstance(other, Series):
   3150             return self._align_series(other, join=join, axis=axis, level
=level,

c:\users\vdbosscj\scipy\pandas-joris\pandas\core\generic.pyc in _align_frame(sel
f, other, join, axis, level, copy, fill_value, method, limit, fill_axis)
   3167                 join_index, ilidx, iridx = \
   3168                     self.index.join(other.index, how=join, level=level,
-> 3169                                     return_indexers=True)
   3170
   3171         if axis is None or axis == 1:

c:\users\vdbosscj\scipy\pandas-joris\pandas\core\index.pyc in join(self, other,
how, level, return_indexers)
   1781                 pass
   1782             else:
-> 1783                 return self._join_multi(other, how=how, return_indexers=
return_indexers)
   1784
   1785         # join on the level

c:\users\vdbosscj\scipy\pandas-joris\pandas\core\index.pyc in _join_multi(self,
other, how, return_indexers)
   1876             raise ValueError("cannot join with no level specified and no
 overlapping names")
   1877         if len(overlap) > 1:
-> 1878             raise NotImplementedError("merging with more than one level
overlap on a multi-index is not implemented")
   1879         jl = overlap[0]
   1880

NotImplementedError: merging with more than one level overlap on a multi-index i
s not implemented
```
