Better error for CSVs not matching table size

[dbp]

If you try to load-table from a CSV source that does not match the number of columns, the error is very unhelpful -- it just says Contents must match header size. It seems like it should be possible to show some sort of diff -- or at least, what was expected, and what was provided!

If header-row is not true, then a count would I guess make sense, but since the default is to have a header-row, showing the header row would likely be useful, since a common workflow is to start from the CSV and use that to create the table. If this were in the error, students could avoid having to open the CSV in some other editor.