[PP-1] [Needs design] Implement design treatment for fallback component

Overview

In #3519168: Handle components provided by ComponentSources EXPLICITLY disappearing — enables deleting JS components that are in use we added a fallback plugin that steps in when a component goes missing.
In that issue @lauriii asked Callum to implement a design for how to convey this to the user both in the preview and in the layers panel.

Proposed resolution

Implement the design once it is ready

User interface changes