QB responses with the error text (by its meaning) but with the HTTP 200 OK status code

Steps to reproduce:
1. Set up a GitLab project to send webhook to a QB server
2. Test this setup in GitLab with an appropriate button "Test"
3. See that the request succeeds but it details shows that it is not

  Response headers:
    Connection: close
    Date: Thu, 04 Jul 2019 11:25:16 GMT
    Content-Type: text/plain
    Vary: Accept-Encoding, User-Agent
    Server: Jetty(9.4.14.v20181114)

  Response body:
    Gitlab webhook setting not defined

and, expectedly, a build is not run..

I suppose QB server should give some 4xx error status code in such cases to simplify diagnostics.