Improve the handling of "empty" CSV rows

[lancespeelmon]

It is common to find a CSV file that has rows at the bottom of the file with only comma (i.e. separator) characters. AFAICT, this seems to be an irregular behavior of both Excel and Google Sheets. The program currently raises some really cryptic exception and it would be much better to just quietly ignore "empty" rows; i.e. rows that only contain separator characters.

Or at a minimum, improve the error message provided to the user.