[JENKINS-15578] Fail gracefully if slave doesn't have permission.

I'm really not clear on how permissions work regarding launching slaves, and whether the `ADMINISTER` permission is really necessary to execute the iOS device `list` binary.

Kohsuke feedback required! :)

But here's a quick patch for [JENKINS-15578](https://issues.jenkins-ci.org/browse/JENKINS-15578), tested on a JNLP OS X slave in a Jenkins with security enabled.