Validate doesn't log errors in downstream sessions

This is an issue when using vorpal in Vantage, but is, I think, fundamental an issue with vorpal itself.

Vorpal's `_commandSetCallback` only logs errors to local sessions, and `validate` returns any failed validations as errors, so if you are trying to validate commands from other sessions (like in Vantage), the command will fail, but without any logging in either the parent or downstream session.

Source
```
vantage
.command('alwaysfail')
.validate(function (args) {
return "Nothing you do will make this command work."
})
```
Example
```
$ vantage [PORT]
vantage~$ alwaysfail
vantage~$
```