CLONE -Missing question types not handled properly on backup (still present or regressed)

Elsewhere in the code, if you have a question of a type that no longer exists (perhaps it was uninstalled) then there is code in place to handle this, so quiz results go on working, etc.

Currently line 499 in question/backuplib.php fails when the current question's type is no longer installed. There may be other locations where missing types are not handled, but this is the first one that is hit.

This is in build 20100311.