dataframe: "rows need to be the same length" error message is not useful

[dustmop]

It's relatively easy to hit this error message, due to adding a blank row, or just consuming invalid csv data, but it needs more context to help the developer fix it. Perhaps displaying row numbers, and a few rows before where the error happens.

It would be nice if DataFrame could automatically fix this problem, but not it is causes too much drift from the reference implementation.