consul force-leave is not validating input

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When filing a bug, please include the following:

### `consul version` for both Client and Server
Client: `0.9.3+ent`
Server: `0.9.3+ent`

### Description of the Issue (and unexpected/desired result)
Consul force-leave command command is not validating input. So if there is a typo or misunderstanding (e.g. using ip instead of node name) consul will silently accept it without any actions. I think it should return "node not found" or something like that.

### Reproduction steps
```
~ $ consul force-leave nonexisting;echo $?
0
```