Thanks for the suggestion.
I will open a new ticket.
I agree that a Truncate option would make sense, but you do not have to close the wizard if this happens today. The wizard dialog is not modal, so just keep it open while you truncate the table manually.
Aha! I hadn't realized that. The fact that the dialog stays on top of the parent window made me think that it was modal.
I was wondering if it would be possible to get a new checkbox in the "Options" section of the Import File screen for "Truncate table before import" when importing to a table that already exists, or perhaps the existing "DROP TABLE" option when importing to a new table could be replaced with a "TRUNCATE TABLE" option when importing to an existing table (in fact that would probably make more sense). Currently if I'm importing to an empty table and there's an error partway during import I have to close down the import wizard (losing my settings), truncate the table manually, then re-start the import wizard from the beginning.