Fix misleading error message about primary key
on first page of table import wizard, and give
additional info about the process by adding
an info label.

Change-Id: I7c511abc512b7ab87321c055057bae4f42e29af1
Reviewed-on: https://gerrit.libreoffice.org/23969Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Lionel Elie Mamane <lionel@mamane.lu>