Fix errorProtoLog error messages

Use Printf instead of Println for formatted strings, and pass
the filename instead of the proto.

Test: cuj_tests
Change-Id: Id261b5a34304a0caa61faa1f3bbc388aacdd25a6
diff --git a/ui/status/log.go b/ui/status/log.go
index 9090f49..d407248 100644
--- a/ui/status/log.go
+++ b/ui/status/log.go
@@ -180,12 +180,12 @@
 func (e *errorProtoLog) Flush() {
 	data, err := proto.Marshal(&e.errorProto)
 	if err != nil {
-		e.log.Println("Failed to marshal build status proto: %v", err)
+		e.log.Printf("Failed to marshal build status proto: %v\n", err)
 		return
 	}
 	err = ioutil.WriteFile(e.filename, []byte(data), 0644)
 	if err != nil {
-		e.log.Println("Failed to write file %s: %v", e.errorProto, err)
+		e.log.Printf("Failed to write file %s: %v\n", e.filename, err)
 	}
 }