title: define constants in cakephp
body: <p>Anybody have an idea, what is the better way to define the error messages in cakephp 3.x,</p>
  
  <p>I am currenlty defing error messages inline like.</p>
  
  <pre><code>$validator
              -&gt;requirePresence('fname', true)
              -&gt;notEmpty('fname')
              -&gt;add('fname', [
                  'length' =&gt; [
                      'rule' =&gt; ['maxLength', 100],
                      'message' =&gt; __('First name cannot be too long.'),
                  ]
      ]);
  </code></pre>
  
  <p>but I want define constants so that it will be helpful to change message in future but I don't know better way to do it.
  Does cake provide any functionality ? </p>