It is not necessary to re-open the region when MOB files cannot be found

In HBASE-17712, we try to re-open the region when store files cannot be found. This is useful for store files in a region, but is not necessary when the MOB files cannot be found, because the store files in a region only contain the references to the MOB files and a re-open of a region doesn't help the lost MOB files.
In this JIRA, we will directly throw DNRIOE only when the MOB files are not found in MobStoreScanner and ReversedMobStoreScanner. Other logics keep the same.