more descriptive error message when assigning dataframe with different index shape to column
http://stackoverflow.com/questions/17432944/python-pandas-error-when-doing-groupby-counts

The below example doesn't make sense, but perhaps the error message could be clearer...

```
df = pd.DataFrame(np.random.randint(0,2,(4,4)),
                  columns=['a', 'b', 'c', 'd'])

In [11]: df
Out[11]:
   a  b  c  d  gr
0  0  1  1  0   1
1  1  1  1  0   1
2  0  1  1  0   1
3  1  1  0  1   0

In [12]: df.groupby(['b', 'c']).count()
Out[12]:
     a  b  c  d  gr
b c
1 0  1  1  1  1   1
  1  3  3  3  3   3

df['gr'] = df.groupby(['b', 'c']).count()
---------------------------------------------------------------------------
ValueError: Buffer dtype mismatch, expected 'Python object' but got 'long'
```
