EC: Support partial reconstruction of a container if there are insufficient nodes

On a small cluster, if there are, say 2 replicas missing from a container, but only 1 node available in the cluster, reconstruction will fail. It is possible to make the situation better by recovering one of the missing replicas and then if more nodes become available later recover the other one, but the code does not do that at the moment. Instead it just fails the reconstruction.

This would only be an issue on small clusters with few spare nodes compared to the EC policy, but we should fix this as an improvement.