## Meta
OS:  macOS
Selenium Version:  3.11, code indicates this should be in master still
Browser:  Chrome

## Expected Behavior
When visiting a page/servlet that does not exist in Grid, a 404 status code should be returned.

## Actual Behavior
A 200 status code is returned for pages that do not exist.

## Steps to reproduce
1) Launch a hub or node
2) Open your browser's network tab
3) Visit localhost:4444/thisdoesntexist

```
Request URL: http://localhost:4444/thisdoesntexist
Request Method: GET
Status Code: 200 OK
```

## More Information
It appears the design for any [non-registered routes](https://github.com/SeleniumHQ/selenium/blob/5457171bd52747eed2ab625607d29670448f4523/java/server/src/org/openqa/grid/selenium/GridLauncherV3.java#L270) to direct to `DisplayHelpServlet`, which [returns the 200](https://github.com/SeleniumHQ/selenium/blob/5457171bd52747eed2ab625607d29670448f4523/java/server/src/org/openqa/grid/web/servlet/DisplayHelpServlet.java#L122).

Considering I typed a page that should not exist, I feel it may be more obvious if a 404 is returned to indicate a problem.

Context: My application was requesting a custom servlet by the wrong name, and became confused when a 200 was returned but the proper action did not occur. A 404 would have very quickly jumped out as a route issue.
package org.openqa.grid.web.servlet;
import static org.hamcrest.CoreMatchers.containsString;
import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertNotNull;
import static org.junit.Assert.assertThat;
import static org.junit.Assert.assertTrue;
import org.hamcrest.CoreMatchers;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.JUnit4;
import org.openqa.grid.web.servlet.console.ConsoleServlet;
import org.openqa.testing.FakeHttpServletResponse;
import org.seleniumhq.jetty9.server.handler.ContextHandler;
import java.io.IOException;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletResponse;
public class DisplayHelpServletTest extends BaseServletTest {
  @Before
  public void setUp() throws ServletException {
    servlet = new DisplayHelpServlet() {
      @Override
      public ServletContext getServletContext() {
        final ContextHandler.Context servletContext = new ContextHandler().getServletContext();
        servletContext.setInitParameter(DisplayHelpServlet.HELPER_TYPE_PARAMETER, "standalone");
        servletContext.setInitParameter(ConsoleServlet.CONSOLE_PATH_PARAMETER, "/wd/hub");
        return servletContext;
      }