Improve feedback display for multiple choice

The feedback for multiple choice is often difficult to understand.

• Feedback for correct answers, is visually is grouped with the incorrect feedback. Especially if the author does not write something like "Yes..." or "Incorrect..." it is not immediately clear what feedback is affirmation and what is corrective.
• If there is no feedback, just the letter is displayed. It is not clear if that is because it is correct or incorrect answer. Some default text should be displayed. (perhaps not needed if all the feedback for incorrect items are grouped separately from correct ones).

  <exercise label="asdf1">
    <statement>
      <p>Which are even numbers?</p>
    </statement>
    <choices randomize="yes">
      <choice>
        <statement>
          <p>1</p>
        </statement>
        <feedback>
          <p>Incorrect. 1 is a bad answer.</p>
        </feedback>
      </choice>
      <choice correct="yes">
        <statement>
          <p>2</p>
        </statement>
        <feedback>
          <p>Correct. 2 is a good answer.</p>
        </feedback>
      </choice>
      <choice>
      <statement>
        <p>3</p>
      </statement>
      <feedback>
        <p>Incorrect. 3 is a bad answer.</p>
      </feedback>
    </choice>
    <choice correct="yes">
      <statement>
        <p>4</p>
      </statement>
      <feedback>
        <p>Correct. 4 is a good answer.</p>
      </feedback>
    </choice>
    <choice correct="yes">
      <statement>
        <p>5</p>
      </statement>
      </choice>
      <choice>
        <statement>
          <p>6</p>
        </statement>
        </choice>
    </choices>
  </exercise>