Quiz: Don't validate units when unit's aren't used in numerical question type

This applies to both export/import.

If the unit handling (unitrole) field of a numerical question is set to 'Units are not used at all' then this should be honoured. Currently it's ignored. When importing or exporting questions an error is displayed. asking you to change Unit handling.
This will confuse lots of folks.

Can we detect whether the question uses Units and if it doesn't set Unit handling to 'Units are not used at all'?