title: How to return specific error code in plumber API?
body: <p>How can we return a specific error code from plumber R API?</p>
  
  <p>We would like to return a 400 status code with a message, e.g. "Feature x is missing.". We read the plumber documentation but could not figure this out.</p>
  
  <p>If we use <code>stop()</code> a 500 Internal Server Error is returned with no specific message.</p>
  
  <pre><code>#' predict
  #' @param x Input feature
  #'
  #' @get /predict
  
  function(x) {
    if (is.na(x)) {
      # error code 400 should be returned
    }
  
   # ... # else continue with prediction
  }
  
  </code></pre>