With the old 4.0-style two phase drop, if the server crashes after the actual WT table drop and before a newer checkpoint has been generated (see below), after restart we will have a state where collection is in mdb_catalog but not backed by any WT tables.
-------- Checkpoint ---- Rename ------------- Actual Drop -------- Server Crash

Therefore we chose to allow NamespaceNotFound errors in replication recovery.

After we've done the new 4.2-style two phase drop, this error should never happen during replication recovery because the actual WT table drop will always happen after a stable checkpoint which includes the mdb_catalog changes.