title: Return custom "invalid grant" error message from GrantResourceOwnerCredentials
body: <p>When authentication fails with in the GrantResourceOwnerCredentials method of <code>SimpleAuthorizationServerProvider</code> it returns;</p>
  
  <pre><code>context.SetError("invalid_grant", "The user name or password is incorrect.");
  </code></pre>
  
  <p>However, I need to return my own JSON response by using a <code>TEntity</code> class, any help would be appreciated.</p>