title: Slick: how to return an error on update if zero row was updated
body: <p>I want to return an error if I try to update an inexistant row in Slick.</p>
  
  <p>my query: </p>
  
  <pre><code>userPromotions.filter(promo =&gt;
    promo.code === code).update(myUpdatedRow)
  </code></pre>
  
  <p>the query I try to do (I have an error cause "rowToUpdate.exist" is a Rep[Boolean] and not a Boolean):</p>
  
  <pre><code>val rowToUpdate = userPromotions.filter(promo =&gt; promo.code === code)
  if(rowToUpdate.exist) rowToUpdate.update(myUpdatedRow)
  else NotFound
  </code></pre>
  
  <p>is there any way to do that in a single query ?</p>