fix: make ErrorWindowI18n serializable

Close #77

src/main/java/com/flowingcode/vaadin/addons/errorwindow/ErrorWindowI18n.java

@@ -22,14 +22,16 @@
 package com.flowingcode.vaadin.addons.errorwindow;
 
 import com.vaadin.flow.function.SerializableFunction;
+import java.io.Serializable;
 
 /**
  * Internationalization object for customizing the component UI texts. An instance with the default
  * messages can be obtained using {@link ErrorWindowI18n#createDefault()}.
  *
  * @author [email protected]
  */
-public class ErrorWindowI18n {
+@SuppressWarnings("serial")
+public class ErrorWindowI18n implements Serializable {
 
   private String caption;
   private String instructions;