fix(js): keep page task errors from killing the event loop (#699)

A script exception or unhandled promise rejection reached the event-loop
drivers as a fatal error, so one throwing page script starved every later
task (pending dynamic scripts, timers). Chrome reports the error and keeps
scheduling. The bounded, quiescent, autonomous, and load-delaying script
pumps now log the page-local error and continue; watchdog termination, the
heap cap, and an exhausted task budget stay fatal.