General information about canaries is available in go/autoroller-canary-bots.
Happens when the skia_try_service in G3 returns an exception. Check the skia_try_service's error logs in go/skia-borg-jobs.
For errors that do not seem to be transient, restarting the borg job has worked in the past:
borg --borg=${BORG_CELL} --user=skia --name=skia_try_service --avoid_parent restarttask 0