should C2M (PreservedObjectHandler#check_existence) always be able to mark a CompleteMoab as missing?

[jmartin-sul]

currently, PreservedObjectHandler#check_existence will decline to validate a CompleteMoab with invalid_checksum status. however, we should considering allowing it to always mark a CompleteMoab as missing (set status to online_moab_not_found). the thinking with the current behavior is that invalid_checksum is a "worse" error than invalid_moab, and so should take precedence since status is a single flat value. however, an entirely missing moab that's in the catalog seems worse than the partial corruption indicated by invalid_checksum, so this is yet another rough edge of our wonky status precedence behavior.

see #1155 for discussion about moving away from the single flat status approach.

#check_existence is actually used by both C2M and M2C, but if it's being called from M2C, it's the result of walking the storage roots for druid directories, so in that scenario the moab directory necessarily exists on disk already. hence this ticket's title.

[jmartin-sul]

noticed while investigating #1480

[andrewjbtw]

@jmartin-sul will verify if this is still an issue

[ndushay]

@jmartin-sul this is another one for you to determine whether it's still an issue, and if so ... at this point, maybe make a new ticket given the refactoring and close this one.

Or if its not still a concert ... just close it.