Fix a whole range of bugs related to corrupted changesets

1. Throw an error if an unknown attrib is referenced -- not necessarily a 'fix', but at least you know why it crashed.
2. Catch errors during prparation of client vars -- if the pad is corrupted, don't even send it to the client, just disconnect
3. Never keep processing a changeset if an error was thrown whilst digesting it -- pull the breaks, disconnect the client and be done

Warning: totally untested