Relevant logs:
    http://cautotest/tko/retrieve_logs.cgi?job=/results/hosts/chromeos4-row5-rack2-host1/51778493-repair/

In the logs, you find these messages:
03/07 01:52:55.428 ERROR|        dev_server:0272| Devserver call failed: "http://172.17.40.22:8082/check_health?", timeout: 6.0 seconds, Error: Call is timed out.
03/07 01:52:55.429 WARNI|         cros_host:0781| Devserver looks unhealthy. Trying another
03/07 01:52:55.430 WARNI|         cros_host:1402| Failed to repair device: local variable 'requested_build' referenced before assignment

The failure about `requested_build` appears to come from this
source code in CrosHost.machine_install():
                    logging.warn('Devserver looks unhealthy. Trying another')
                    update_url, devserver = self._stage_image_for_update(
                            requested_build)
                    logging.debug('New Update URL is %s', update_url)

A cursory glance through machine_install() shows that there are code
paths that do not set `requested_build`, so the error is probably an
edge case.

This problem happened at a time when many devservers were unavailable
due to overload; the exception probably can't happen in more
ordinary circumstances.

I don't _think_ that this symptom is severe, so I'm calling it "P2".