[V4] Fast Refresh should dismiss runtime errors

Created by: gaearon

1. Type foo.bar() inside a component
2. See runtime error overlay
3. Change that to foo.baz() and save

Expected: see the foo.baz() error overlay instead. Actual: keep seeing the foo.bar() overlay with the new error being "added" (the screen says 1 of 2 errors).

In other words, when we do a Fast Refresh, we should always be dismissing the runtime overlay first.

This is because the previous error is stale by the time you save the code and likely irrelevant.