Be specific about more indexes or values
The generic "are not equal" really frustrated me just now when someone was conveying their issue.

This specific two errors lets me know which way around, although I believe having more indexes than data shows a place in the api for a default value (possibly per-index) which would allow this to later be reduced to the case I feel I fully understand of more data than indexes, which I think probably is unsolvable generally.

I can see that both have problems, but I still think it's more important to be specific about which side is mismatched.

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