title: Doctrine2 finding an entity that doesn't exist
body: <p>What happens if in Doctrine2 I do the following with an entity manager.</p>
  
  <pre><code>$object = $em-&gt;find( 'MyModel\User', -999 );
  </code></pre>
  
  <p>Is object false, is it a User object, does an exception get thrown? Basically, how can I test if a row exists in the database based on some criteria?</p>