[autotest] Fix label detector to handle unreachable hosts.

TEST=Ran the detector.
BUG=chromium:219331

Change-Id: I457077634c78a7dd377e35e672edc27c12b4129d
Reviewed-on: https://chromium-review.googlesource.com/223731
Commit-Queue: Prashanth B <beeps@chromium.org>
Tested-by: Prashanth B <beeps@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
2 files changed