Collapsible Error Dialogs for the Ecosystem

Spinoff from https://github.com/facebook/react/pull/15797#issuecomment-504782329. Both in React Native and Create React App, redboxes are full screen. But in React, most errors are recoverable. Even with accidental runtime crashes it's useful to look "underneath" to see whether your boundary worked as expected, and to have an idea of the end user experience. We could solve this with a collapsed-by-default floating error panel that just shows a list of messages. You can click to expand. We could also use this as an opportunity to unify the RN and web designs.