Occasional rate limit issue with Tasks API

[davidlang42]

Need to use backoff code for all "Tasks" calls:

const BACKOFF_WAIT_START = 1; // ms
const DEFAULT_MAX_TOTAL_WAIT = 1000; // ms

function runWithBackoff(func, max_total_wait) {
  max_total_wait = max_total_wait ?? DEFAULT_MAX_TOTAL_WAIT;
  let ms = BACKOFF_WAIT_START;
  let total = 0;
  while (true) {
    try {
      return func();
    } catch(err) {
      if (total < max_total_wait) {
        Utilities.sleep(ms);
        total += ms;
        ms = ms * 2;
      } else {
        throw new Error("After waiting " + total + "ms: " + err.message);
      }
    }
  }
}