title: How to customize error message of OAuthAuthorizationServerProvider?
body: <p>We are using the <code>OAuthAuthorizationServerProvider</code> class to do authorization in our ASP.NET Web Api app.</p>
  
  <p>If the provided username and password is invalid in <code>GrantResourceOwnerCredentials</code>, the call</p>
  
  <pre><code>context.SetError( "invalid_grant", "The user name or password is incorrect." );
  </code></pre>
  
  <p>Produces the following Json result:</p>
  
  <pre><code>{
      "error": "invalid_grant",
      "error_description": "The user name or password is incorrect."
  }
  </code></pre>
  
  <p>Is there any way to customize this error result?<br>
  I would like to make it consistent with default error message format used in other parts of the API:</p>
  
  <pre><code>{
      "message": "Some error occurred."
  }
  </code></pre>
  
  <p>Is this possible to achieve with the <code>OAuthAuthorizationServerProvider</code>?</p>