[Fiber] Error handling

[gaearon]

Filing this as a todo list for myself.

• [Fiber] Make error handling more resilient #8210 After more scheduling tests are in place, add tests for uncaught errors in different roots
• [Fiber] "Task" priority for error boundaries and batched updates #8193 Remove recursion
• [Fiber] Force rerender a failed tree #8227 Choose and implement some sensible behavior for fataled roots (eg visiblity: hidden)
• [Fiber] Add more tests and fix an issue with incremental error handling #8212 Look at batchedUpdates ([Fiber] Use synchronous scheduling by default #8127 (comment), [Fiber] Use synchronous scheduling by default #8127 (comment))

[sebmarkbage]

Do you want to add "Error handling when an error is thrown in detachRef callbacks" from the umbrella? Maybe we're happy with how it works already. However, I believe this case might be problematic: return <ErrorBoundary><div ref={n => n.width} /><div ref={n => this.b = n} /></ErrorBoundary>; since during unmount the first will throw, and the second won't be cleaned up.