Better status code and error message for when a component isn't found

[mheiber]

Summary:

When a page can't render because a component isn't found, we should get an error message saying that the page can't render because a component isn't found.

Steps to reproduce

Add a new component reference to editable_components for a component that doesn't exist
Try to create a page
Result: 404 with no message

Expected result: better error message like "component not found". Maybe a 500 would be a better status code.

Thanks for your help!

[pfeytser]

This is fixed in cases where the data is 404-ing, but needs to be addressed for render.

[nelsonpecora]

@jonwinton this is fixed for rendering by #544 yeah?

[jonwinton]

Yeah