dillon1000/react

Error recovery should have task priority

This uncovered a separate issue where some errors were being unscheduled
and rescheduled multiple times before flushing. Turns out we need to
check both a fiber and its alternate when determining if it represents a
failed unit of work. I didn't notice this before because we were
scheduling a new update on *every* boundary at the end of the commit
phase, not just the ones that captured an error during that commit. I
updated the unit tests to catch this in the future.