Check to make sure credential is not null in SSH command interface

Summary: This adds a check to make sure the credential exists when loading it in the Drydock SSH interface. This effectively turns a fatal error (calling a method on a non-object) into a catchable exception.

Test Plan: Had a badly configured resource, saw the exception appear instead of daemon fataling.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11530