cmd/watchflakes: ignore failed builds without output properties

It turns out to be possible¹ for a build to fail early enough that it
doesn't get a chance to produce output properties, and not get marked
as an infrastructure failure (even though it should've been). There's
not much choice but to ignore such builds.

¹ For example, https://ci.chromium.org/b/8752293940218446529. It failed
  very early while running bbagent, before golangbuild was even started.

For golang/go#65353.

Change-Id: I14782f41e7a390380a32272b58cda385c91cb881
Reviewed-on: https://go-review.googlesource.com/c/build/+/579299
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

cmd/watchflakes/luci.go

@@ -317,7 +317,7 @@ func (c *LUCIClient) ReadBoard(ctx context.Context, dash *Dashboard, since time.
 				}
 				if commit == "" {
 					switch b.GetStatus() {
-					case bbpb.Status_SUCCESS, bbpb.Status_FAILURE:
+					case bbpb.Status_SUCCESS:
 						log.Fatalf("empty commit: %s", buildURL(id))
 					default:
 						// unfinished build, or infra failure, ignore