title: Recursive search values by key
body: <p>I have a JSON like this:</p>
  
  <pre><code>{ 
    "A": { "error": null },
    "B": { "C": {"error": "error string"}},
    "C": { "D": {"error": null}},
    "D": { "error": "err str"}
  }
  </code></pre>
  
  <p>end I want to find all values of <strong>error</strong> keys which are not <strong>null</strong>.</p>
  
  <p>For my example it should return </p>
  
  <pre><code>"error string"
  "err str"
  </code></pre>
  
  <p>How can I do it? Is it possible with <strong>jq</strong>?</p>