title: Promise.then() firing on SequelizeUniqueConstraintError
body: <p>I am a little new to promises and Sequelize. I am trying to insert a new user to my SQLite-database using constraint. In the <code>User</code>-model I have flagged the <code>email</code>-property as <code>unique</code>, however when this constraint is not met the code inside the <code>.then()</code>-block is still executed. </p>
  
  <p>Code:</p>
  
  <pre class="lang-js prettyprint-override"><code>User.create({
     email: uname,
     password: psw
  })
     .then(console.log('success'))
     .catch((err) =&gt; { console.log(err); });
  </code></pre>
  
  <p>Output: </p>
  
  <pre><code>Success
  Executing (default): INSERT INTO `users` ...
  { SequelizeUniqueConstraintError: Validation Error ... }
  </code></pre>
  
  <p>No entry is of course added to the DB.</p>
  
  <p>Expected output (when inserting unique value):</p>
  
  <pre><code>Executing (default): INSERT INTO `users` ...
  success
  </code></pre>
  
  <p>Expected output (when inserting duplicate value):</p>
  
  <pre><code>Executing (default): INSERT INTO `users` ...
  { SequelizeUniqueConstraintError: Validation Error ... }
  </code></pre>
  
  <p>Any help would be <em>very</em> welcome, thank you!</p>