Request: Better error message for missing columns in aggregate

[tsoernes]

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

[MarcoGorelli]

Thanks @tsoernes - do you want to work on a pull request? https://pandas.pydata.org/docs/development/contributing.html

[tsoernes]

I'm sorry but I'm unable to contribute at this moment.