Don't try to survive scanning problems

This change does two things:
 * Fail scans instead of becoming confused.
   (The earlier solution preserved _mostly_ correct
   functionality on _most_ pages, but creates unsolvable
   problems on others - so we give up on this approach for now.)
 * Add minimal destroy functionality
Done in https://github.com/hypothesis/dom-text-mapper/commit/4ff6cf4da26b7d62dde6497e428e9f7fbcf038df