Rob Allen has a quick post to his site talking about some of the improved error handling that's been updated in the latest version of the Slim microframework to help make reporting issues easier in multiple contexts.

From RC1 of Slim 3, we have improved our error handling. We've always had error handling for HTML so that when an exception occurs, you get a nice error page [...] However, if you're writing an API that sends and expects JSON, then it still sends back HTML. [...] At least we set the right Content-Type and status code! However, this isn't really good enough. We should send back JSON if the client has asked for JSON. Until RC1, the only way to do this was to register your own error handler.

With Slim 3 the framework handles things more correctly based on the value of the "Accept" header sent along with the request. This value is checked and, if it references JSON or XML, the error message is translated either giving the default output or reporting back for the "notFound" and "notAllowed" error types.