ERR: Better error message for missing columns in aggregate
More descriptive SpecificationError message that reports to user non-existing columns causing error

- [x] closes #32755
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

merge master to fix the docs issue @jreback Made changes and green thanks @JDTruj2018 