## 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.
setStatus(SC_SEE_OTHER);
    setHeader("Location", s);
  }
  public void setStatus(int i) {
    this.status = i;
  }
  public void setStatus(int i, String s) {
    throw new UnsupportedOperationException();
  }
  public String getCharacterEncoding() {
    throw new UnsupportedOperationException();
  }
  public String getContentType() {
    return getHeader("Content-Type");
  }
  public ServletOutputStream getOutputStream() {
    return servletOutputStream;
  }
  public PrintWriter getWriter() {
    return printWriter;
  }
  public void setCharacterEncoding(String s) {
    String type = getHeader("content-type");
    setHeader("content-type", type + "; charset=" + s);
  }
  public void setContentLength(int i) {
    setIntHeader("content-length", i);
  }
  @Override
  public void setContentLengthLong(long len) {