Make device_status_check complain if there are no available devices.

BUG=

Review-Url: https://codereview.chromium.org/2104413002
Cr-Commit-Position: refs/heads/master@{#403309}

build/android/buildbot/bb_device_status_check.py

@@ -409,6 +409,8 @@ def main():
                       and not _IsBlacklisted(status['serial'], blacklist))]
 
   # If all devices failed, or if there are no devices, it's an infra error.
+  if not live_devices:
+    logging.error('No available devices.')
   return 0 if live_devices else exit_codes.INFRA