erlang function to add a shard with simple error handling

gistfile1.erl

add_shard(Master, Slave) ->
    ErrorMsg = "unable to add shard",
    case ?NEW_SHARD_KEY of
        {error, _Msg} -> 
            {error, ErrorMsg};
        ShardId ->
            case ?SHARD_KEY_FOR_MEMBERS(ShardId) of
                {error, _Msg} -> 
                    {error, ErrorMsg};
                ShardKey ->
                    Members = ?ENCODE_MASTER_SLAVE(Master,Slave),
                    case ?REDIS(["SET",ShardKey,Members]) of
                        {error, _Msg} -> {error, ErrorMsg};
                        _AnythingElse -> {ok, ShardId}
                    end
            end
    end.