Return 404 instead of undefined variable error when trying to edit a non-existent form

Summary: E.g. https://phab-01.wmflabs.org/transactions/editengine/transactions.editengine.config/view/13/

Test Plan:

• Go to /transactions/editengine/transactions.editengine.config/view/1000000/
• Observe error
• Apply patch
• Observe 404

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D15307