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