create new error module, which displays errors before migration

scm-webapp/src/main/java/sonia/scm/boot/StaticResourceServlet.java

@@ -0,0 +1,39 @@
+package sonia.scm.boot;
+
+import com.github.sdorra.webresources.CacheControl;
+import com.github.sdorra.webresources.WebResourceSender;
+import com.google.common.annotations.VisibleForTesting;
+import sonia.scm.util.HttpUtil;
+
+import javax.inject.Singleton;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import java.io.IOException;
+import java.net.MalformedURLException;
+import java.net.URL;
+
+@Singleton
+public class StaticResourceServlet extends HttpServlet {
+
+  private final WebResourceSender sender = WebResourceSender.create()
+    .withGZIP()
+    .withGZIPMinLength(512)
+    .withBufferSize(16384)
+    .withCacheControl(CacheControl.create().noCache());
+
+  @Override
+  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
+    URL resource = createResourceUrlFromRequest(request);
+    if (resource != null) {
+      sender.resource(resource).get(request, response);
+    } else {
+      response.setStatus(HttpServletResponse.SC_NOT_FOUND);
+    }
+  }
+
+  private URL createResourceUrlFromRequest(HttpServletRequest request) throws MalformedURLException {
+    String uri = HttpUtil.getStrippedURI(request);
+    return request.getServletContext().getResource(uri);
+  }
+}