Error uploading CiviCRM contacts. Can I revert?
To revert an import done with the import functionality at Contacts > Import Contacts, you will need to restore from your backed up database file. As far as I know, there is no automatic "undo" on imports.

To your point, it might be worthwhile to add a new warning to the import "Are you sure" verification message that pops up right before the import fires:


For example, "Are you sure you want to import now? Be sure you have backed up your database as there is no 'undo' for this." (Update: I submitted a patch so if the core team agrees this will be added in future versions)