More descriptive error message for missing docker image.
More descriptive error message for missing docker image.

Tested:
  Ran a packetimpact test after `docker image rm` and examined the message.
PiperOrigin-RevId: 308370603

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor