dillon1000/react

If an error is thrown and there's no nextUnitOfWork, it's fatal error

This can only be caused by a bug in the renderer, but we should handle
it gracefully anyway.

Added a TODO to change capturedErrors and failedBoundaries so that they
are sets of instances (stateNodes) rather than fibers, to avoid having
to check the alternate. (This outside the scope of this PR.)