Currently, HFE commands reply with empty array if the key does not
exist. Though, non-existing key and empty key is the same thing.
It means fields given in the command do not exist in the empty key.
So, replying with an array of 'no field' error codes (-2) suits better
to Redis logic. e.g. Similarly, `hmget` returns array of nulls if the
key does not exist.
After this PR:
```
127.0.0.1:6379> hpersist missingkey fields 2 a b
1) (integer) -2
2) (integer) -2
```
Ozan Tezcan
4aa25d042c