title: exception for non existing parameter in FLASK
body: <p>I have a form that sends parameters.
  In my form I have a checkbox.
  If my checkbox is not checked then I will not get any parameters.</p>
  
  <p>If in my module I have :</p>
  
  <pre><code>var = request.form['mycheckbox']
  </code></pre>
  
  <p>and if my checkbox is not checked (the parameter is not passed)</p>
  
  <p>Then in debug mode I get the error message :</p>
  
  <blockquote>
    <p>Bad Request The browser (or proxy) sent a request that this server
    could not understand.</p>
  </blockquote>
  
  <p>Nothing tells me about what the error is.</p>
  
  <p>I prevented the exception by using :</p>
  
  <pre><code>    try:
          var=request.form['checkbox']
      except:
          var=None
  </code></pre>
  
  <p>But can I not change the behavior of how Flask handles this case ?</p>