Lock books downloaded

Books downloaded from Bloom library should be editable
in localize mode, not author mode, even if they were
accidentally uploaded unlocked.
They won't be able to use the book because there isn't one there that Bloom can find. If they try they will get some error message saying the book folder has no (or multiple) htm files (cf BL-678).
Should be very rare...indicates a problem with the uploading of the book. Not sure whether it's worth doing more than a minimal don't-crash check here.