title: rails 4 validates presence and length on the same parameter
body: <p>Is it a good idea to having validates like this:</p>
  
  <pre><code>validates :serialnumber, presence: true, length: {7..20}, format: {with: /\d{7,20/}
  </code></pre>
  
  <p>As you see it generates three errors if I don't type serialnumber.
  I would like to see only one error.
  If I type nothing, I would like to see 'serial number is required' only.
  If I type <code>123ABC</code> I would like to see 'wrong length' only
  And if I type <code>123-ABC-123</code> I would like to see 'wrong format' only</p>
  
  <p>How to do it?</p>
  
  <p>Regards</p>