magma-kvstore no error check for the Key/Seqno index scanning.

Code review spotted that Magma-KVStore (the KV-engine code which drives magma) isn't doing an error check when scan/iterator loop ends. The assumption seems to be that the loop terminated successfully.

If there was a system error during scanning, e.g. a disk was unmounted, KV will end a DCP backfill early and report success.

This can lead to DCP consumers (e.g. the data replica) having a gap in the data, e.g. the disk backfill ends and switches to memory, yet we missed data out because of the undetected failure.