More details in error message when valid values check fails

[annakrystalli]

Currently when check_tbl_values fails it only gives the index of rows affected. This is not at all useful really, nor would returning the whole row.

It would be better to:

• First check whether there are outright invalid values in any columns
• some way of describing remaining invalid combinations of valid values.

This is trickier and maybe just the index will suffice.

Care require not to add too much additional computation load though