Request: Better error message for missing columns in aggregate
When attempting to aggregate a non-existing column, this error is raised, which is not very descriptive: `SpecificationError: nested renamer is not supported`
```
In [39]: pd.DataFrame([[1, 2]], columns=['A', 'B']).groupby('A').agg({'B': 'mean'})
Out[39]: 
   B
A   
1  2

In [40]: pd.DataFrame([[1, 2]], columns=['A', 'B']).groupby('A').agg({'B': 'mean', 'non-existing': 'mean'})
Out[40]:
...
SpecificationError: nested renamer is not supported
```