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

src/applications/drydock/interface/command/DrydockSSHCommandInterface.php

@@ -16,6 +16,12 @@ final class DrydockSSHCommandInterface extends DrydockCommandInterface {
       ->needSecrets(true)
       ->executeOne();
 
+    if ($credential === null) {
+      throw new Exception(pht(
+        'There is no credential with ID %d.',
+        $this->getConfig('credential')));
+    }
+
     if ($credential->getProvidesType() !==
       PassphraseCredentialTypeSSHPrivateKey::PROVIDES_TYPE) {
       throw new Exception('Only private key credentials are supported.');