Silence error during microfactory trace import

This may happen if there's been another call to microfactory before
we've read the log file. Just ignore the error and continue.

Test: Run `while true; do get_build_var TARGET_PRODUCT; done` in parallel
Change-Id: I670144f4637281ebf1299dbd9151dd9819c363e3
diff --git a/ui/tracer/microfactory.go b/ui/tracer/microfactory.go
index 320d9d8..acb9be4 100644
--- a/ui/tracer/microfactory.go
+++ b/ui/tracer/microfactory.go
@@ -28,7 +28,7 @@
 
 	f, err := os.Open(filename)
 	if err != nil {
-		t.log.Println("Error opening microfactory trace:", err)
+		t.log.Verboseln("Error opening microfactory trace:", err)
 		return
 	}
 	defer f.Close()