Comment 4 for bug 86202

I suggest that if there are already four primary partitions, clicking the button to add a new partition should open an error alert with:
* primary text 'Sorry, adding another partition to this disk isn’t possible when it has four “primary partitions”.'
* secondary text "If you delete any of the primary partitions, you can then add as many partitions as you wish."
* five radio buttons, the first to "Leave all partitions as they are", then one each to "Delete ____" one of the primary partitions.
* "Cancel" and "Delete" buttons, where the "Delete" button is insensitive when "Leave all partitions as they are" is selected.