Don't show `help requested` errors as errors
Don't show `help requested` errors as errors

This turns the output from:

```
❯ vtctlclient -server "localhost:15999" ListAllTablets -h
Usage: ListAllTablets <cell name1>, <cell name2>, ...

Lists all tablets in an awk-friendly way.

ListAllTablets Error: rpc error: code = Unknown desc = flag: help requested
E0528 19:52:46.564223   77260 main.go:76] remote error: rpc error: code = Unknown desc = flag: help requested
❯ echo $?
1
```

into:

```
❯ vtctlclient -server "localhost:15999" ListAllTablets -h
Usage: ListAllTablets <cell name1>, <cell name2>, ...

Lists all tablets in an awk-friendly way.

❯ echo $?
0
```

Signed-off-by: Andrew Mason <8a240ff75aa1467ba5eba4db0c8200b61262547e@slack-corp.com>

vitessio/vitess,mahak/vitess,tinyspeck/vitess,vitessio/vitess,tinyspeck/vitess,mahak/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,tinyspeck/vitess,vitessio/vitess,tinyspeck/vitess,tinyspeck/vitess,tinyspeck/vitess,vitessio/vitess,tinyspeck/vitess,mahak/vitess,mahak/vitess,vitessio/vitess