Print the output from RBE bootstrap shutdown

The output from bootstrap would contain the a one-line summary of RBE
download / upload stats.

Bug: b/183007125
Change-Id: I90082a8433504e40bbf92992c3c8d1d656c49429

ui/build/rbe.go

@@ -112,9 +112,15 @@ func startRBE(ctx Context, config Config) {
 
 func stopRBE(ctx Context, config Config) {
 	cmd := Command(ctx, config, "stopRBE bootstrap", rbeCommand(ctx, config, bootstrapCmd), "-shutdown")
-	if output, err := cmd.CombinedOutput(); err != nil {
+	output, err := cmd.CombinedOutput()
+	if err != nil {
 		ctx.Fatalf("rbe bootstrap with shutdown failed with: %v\n%s\n", err, output)
 	}
+
+	if len(output) > 0 {
+		fmt.Fprintln(ctx.Writer, "")
+		fmt.Fprintln(ctx.Writer, fmt.Sprintf("%s", output))
+	}
 }
 
 // DumpRBEMetrics creates a metrics protobuf file containing RBE related metrics.