chromeos: Ensure cros_run_test's exit code is propagated if it's non-zero.

eg: https://chromium-swarm.appspot.com/task?id=4527b399d8f02d10
Something went wrong there, and cros_run_test return non-zero, but no
failed Tast tests were found, so the test runner returned zero.

This'll prevent that from happening.

Bug: 947531
Change-Id: Ie43912771820f3920c30b82f82d6d54555868164
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1638777Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665223}

build/chromeos/test_runner.py

@@ -366,6 +366,11 @@ class TastTest(RemoteTest):
 
     if not suite_results.DidRunPass():
       return 1
+    elif return_code:
+      logging.warning(
+          'No failed tests found, but exit code of %d was returned from '
+          'cros_run_test.', return_code)
+      return return_code
     return 0