fix(command): use unix style error messages

These simple etcdctl utilities should try and follow the unix philosophy
as much as possible. Return short useful errors, exit with 1.

Example from rm:

```
$ rm third_party
rm: third_party: is a directory
$ echo $?
1
```
Merge pull request #65 from philips/follow-unix-error-messages

fix(command): use unix style error messages