title: Symfony Validation Error Message for Missing Required Form Field
body: <p>I have a working Form, it has a required field that needs to be notBlank:</p>
  
  <pre><code>/**
   * @Assert\NotBlank
   */
  private $field1 = '';
  </code></pre>
  
  <p>If I specify this field in the request, but leave the field empty, I get this response:</p>
  
  <pre><code>{
      "code":400,
      "message":"Validation Failed",
      "errors":{
          "children":{
              "field1":{
                  "errors":["Field should not be blank"]
              }
          }
      }
  }
  </code></pre>
  
  <p>If I omit this field from the request, I get this response:</p>
  
  <pre><code>{
      "code":400,
      "message":"Validation Failed",
      "errors":{
          "errors":["Field should not be blank"]
      }
  }
  </code></pre>
  
  <p>Is there some built-in Symfony logic somewhere that I can use to make the second example match the first example?</p>
  
  <p>[edit] Was using Symfony 2.5 - now updated to Symfony 2.8.3, same problem.</p>