title: symfony2 difference between error.message and error.messageKey
body: <p>I am implementing a simple custom login form. I am following two different example , the official one <a href="http://symfony.com/doc/current/cookbook/security/form_login_setup.html" rel="nofollow">http://symfony.com/doc/current/cookbook/security/form_login_setup.html</a> and this other one <a href="https://knpuniversity.com/screencast/symfony2-ep2/logout#play" rel="nofollow">https://knpuniversity.com/screencast/symfony2-ep2/logout#play</a> which is substantially the same but with some differences.
  Giving a look at the login.html.twig of the two examples, one of the differences is in the error message reporting where the first reports </p>
  
  <pre><code>&lt;div class="error"&gt;{{ error.message|trans }}&lt;/div&gt;
  </code></pre>
  
  <p>while the other reports </p>
  
  <pre><code>div class="error"&gt;{{ error.messageKey|trans(error.messageData, 'security') }}&lt;/div&gt;
  </code></pre>
  
  <p>Please here's my  question : what's the difference between "error.message" and "error.messageKey" and what do error.messageData means in the second example ?</p>