Add "PARTIAL" snapshot status

[imotov]

Currently even if some shards of the snapshot are not snapshotted successfully, the snapshot is still marked as "SUCCESS". Users may miss the fact the there are shard failures present in the snapshot and think that snapshot was completed successfully (#5657 and #5742). A new "PARTIAL" snapshot status should be added to provides a quick indication that the snapshot was only partially successful.

[s1monw]

@imotov is this a bug too? It seems to be tagged as 1.1.1 which I don't think it should be tagged with from the labels that I see.

[imotov]

Oops, got carried away. That's definitely 1.2 and above feature.