Error handling for missing page templates

[joshvc]

• Add a new entry in cardboard.yml and seed it
• Forget to create the page template
• Edit the page in cardboard, then try to visit the page

The action 'your_page' could not be found for PagesController

It would be nice if we could intercept this error and render something more useful- more like the Missing Template error, since that's what's actually happening.