add: api servlet, 404 and 500 page

dashboard/src/main/scala/ScalatraBootstrap.scala

@@ -4,6 +4,7 @@ import javax.servlet.ServletContext
 
 class ScalatraBootstrap extends LifeCycle {
   override def init(context: ServletContext) {
-    context.mount(new ReleaseDashboardServlet, "/*")
+    context.mount(new StaticHtmlServlet, "/*")
+    context.mount(new DataApiServlet, "/api/*")
   }
 }

dashboard/src/main/webapp/error.html

+<div>
+    <h1>500 Server Error</h1>
+</div>
\ No newline at end of file

dashboard/src/main/webapp/notFound.html

+<div>
+    <h1>404 Not Found</h1>
+</div>
\ No newline at end of file