title: Customise the error response when invalid JSON data is submitted
body: <p>I am sending an invalid JSON that contains just <code>{</code> to my Flask app that validates the JSON and returns its result.</p>
  <p>I got the following error by submitting the command through cURL.</p>
  <pre><code>curl -i -X POST -H &quot;Content-Type:application/json&quot; -d @/Users/xxxx/yyyy/test.json http://0.0.0.0:5000/validate
  </code></pre>
  <pre><code>HTTP/1.0 400 BAD REQUEST
  Content-Type: text/html
  Content-Length: 221
  Server: Werkzeug/0.16.0 Python/3.7.3
  Date: Wed, 05 Aug 2020 11:09:48 GMT
  
  &lt;!DOCTYPE HTML PUBLIC &quot;-//W3C//DTD HTML 3.2 Final//EN&quot;&gt;
  &lt;title&gt;400 Bad Request&lt;/title&gt;
  &lt;h1&gt;Bad Request&lt;/h1&gt;
  &lt;p&gt;Failed to decode JSON object: Expecting property name enclosed in double quotes: line 1 column 2 (char 1)&lt;/p&gt;
  </code></pre>
  <p>Normally, if the data is a valid JSON then there is a response from the server, but in this case, it seems that the app didn't make any reaction.</p>
  <p>My questions are</p>
  <ul>
  <li>What exactly returns this message?</li>
  <li>Is it possible to customise the response? I'd like to customise the message in particular.</li>
  </ul>