A few notes below on some relevant implementation details.

First in server/hosts/servo_repair.py there's this TODO:
        # TODO(jrbarnette):  We want a verifier for whether there's
        # a working USB stick plugged into the servo.  However,
        # although we always want to log USB stick problems, we don't
        # want to fail the servo because we don't want a missing USB
        # stick to prevent, say, power cycling the DUT.
        #
        # So, it may be that the right fix is to put diagnosis into
        # ServoInstallRepair rather than add a verifier.

Implementation-wise, I'm pretty sure that the design requires that
we add code into ServoInstallRepair that checks for the USB stick
presence up front.  Then, if the subsequent chromeos-install command
fails, we need post hoc diagnosis that can figure out whether the
failure was because of the USB stick.  Then, the repair failure can
report "bad USB stick" in the status.log, which is the first place
the lab techs should look when troubleshooting.