title: Symfony choice field type reports "This value is not valid" when submitting an invalid option. How do I change this?
body: <p>I've noticed that when using the Symfony 2.3 choice field type, if I try to submit an invalid option (by manually changing the value of an option), symfony reports a form error on that field that says "This value is not valid". </p>
  
  <p>However, I see no option to change this message in the choice field type class. I have not validation constraints set up for this field either. </p>
  
  <p>Where is this error message coming from, and how do I change it?</p>