disk/luks2: Continue trying all keyslots even if there are some failures

disk/luks2: Continue trying all keyslots even if there are some failures

luks2_get_keyslot() can fail for a variety of reasons that do not necessarily
mean the next keyslot should not be tried (e.g. a new kdf type). So always
try the next slot. This will make GRUB more resilient to non-spec json data
that 3rd party systems may add. We do not care if some of the keyslots are
unusable, only if there is at least one that is.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>