## 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 com.google.common.io.ByteStreams;
import org.openqa.grid.common.GridRole;
import org.openqa.grid.web.servlet.console.ConsoleServlet;
import org.openqa.selenium.internal.BuildInfo;
import org.openqa.selenium.json.Json;
import org.openqa.selenium.json.JsonOutput;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.stream.Collectors;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
 * Displays a somewhat useful help signpost page. Expects {@link #HELPER_TYPE_PARAMETER} to be
 * set as a servlet context init parameter with a value of "hub", "node", or "standalone"
 */
public class DisplayHelpServlet extends HttpServlet {
  private static final long serialVersionUID = 8484071790930378855L;
  public static final String HELPER_TYPE_PARAMETER = "webdriver.server.displayhelpservlet.type";
  private static final String HELPER_SERVLET_TEMPLATE = "displayhelpservlet.html";
  private static final String HELPER_SERVLET_ASSET_PATH_PREFIX = "/assets/";
  private static final String HELPER_SERVLET_RESOURCE_PATH = "org/openqa/grid/images/";
  private static final String HELPER_SERVLET_TEMPLATE_CONFIG_JSON_VAR = "${servletConfigJson}";
  private final class DisplayHelpServletConfig {