Stackdriver "info" level log entry:
Script delayed (X.0 min) waiting for quota
I had a couple of "info" level log entries for my "Form Submit" function from my Google add-on that I'd never seen before. I'm assuming that the quota came back, and that the function ran. One was a 2 minute wait and the other a 3 minute wait.
I guess that if it failed that I'd get a different error.