Bug 978837 - Don't show stack trace if --ssh executable not found

rhc app ssh --ssh /path/to/nowhere should return a short error
(not found or not executable) to the user instead of a stack trace.
Merge pull request #418 from ncdc/bz978837-fix-ssh-no-such-file-or-directory-exception

Merged by openshift-bot
Add net-ssh-multi as dependency on F19+
Merge pull request #419 from kraman/net_ssh_multi_dependency_fix

Merged by openshift-bot