Better AddBatch error returns

[arnaucube]

Currently AddBatch returns an array of invalids indexes, and if when adding the given key-values there is any errors, the method will return err=nil, while returning an array of the index of the invalid keys.
Initially this was done to not break the process when an error was found (like for example there can be one incorrect key but the rest are fine, the AddBatch should complete the rest of keys, and indicate that there is one key which was invalid).
This is not ideal, and should be updated to return the errors for each invalid key, allowing to know which was the cause of that invalid entry.