Use `column-name` instead of `column-number`

[Stephen-Gates]

If a table has a header row, use column-name instead of column-number to provide a more meaningful error message.

(Or perhaps this is something to be done by the client?)

[roll]

@Stephen-Gates
I think in spec notes we could allow implementations to insert column name instead of column number when possible. On spec level I think we can't handle it because for general tabular data there is no guarantee to have column names.

Other question that column in error messages may be could be better than column-number in this case. But I think that's not really critical.