text: Title: eve crashes on malformed sort parameters
  Body: When sending a malformed sorting parameter to eve, it should return `400` for bad request.
  Instead, eve crashes and returns a `500`.
  
  This can be reproduces with the demo app:
  
  This works:
  
  ```sh
  curl "http://eve-demo.herokuapp.com/people?sort=firstname"
  ```
  
  This crashes with `500`, instead of returning `400`:
  
  ```sh
  curl "http://eve-demo.herokuapp.com/people?sort='firstname'"
  ```
  
  Is there any way to fix this?