composability & referential stability in useErrorBoundary

[brainlessbadger]

For your consideration, two changes to useErrorBoundary to bring it more in line with other hooks

• ensure that useErrorBoundary can be called multiple times in the same component independently, like all other hooks do
• ensure referential stability of the reset callback, like useState does

[coveralls]

Coverage remained the same at 100.0% when pulling 14a2604 on brainlessbadger:error-boundary-changes into e8a4c46 on preactjs:master.