title: spring oauth2 custom login error
body: <p>I have got problem to define custom error message when login failed, so now when my login fail i get http 400 with payload: </p>
  
  <pre><code>{"error":"invalid_grant","error_description":"Bad credentials"}
  </code></pre>
  
  <p>How to customize this message, and return own json?</p>
  
  <p>I am using spring boot (1.3.2.RELEASE) and spring security OAuth2 (2.0.8.RELEASE). </p>