flash-device.sh leaves device in fastboot if ubuntu-device-flash fails

[Max Brustkern]

If ubuntu-device-flash fails to flash (which it does, often, and frequently for transient network reasons,) the script will stop and the device will just sit in fastboot mode. The script is clever enough to pick up the next run in fastboot mode (woohoo!) but we should probably have better handling of u-d-f failing. If I remember right, it doesn't provide failure information via exit status, so perhaps we should either scrape the output or put a timeout around the process of waiting for the device to boot.