The target table should be re-created if the merge table operation failed

When the MergeTable processor runs in the 'sync mode', the TableMergeSyncSupport.doSync() will be called,
and it will do following steps:
1. create a temporary "sync" table by using 'create table ... like the-target-table'
2. insert data into the sync table
3. drop the target table
4. change the sync table to external table
5. rename the sync table to the target table
6. change the table back to managed table

but when an error occurs in step 4, 5 and 6 above, the target table will not exist
and the feed will not run successfully anymore since the step 1 will always fail because the target table has been deleted.