title: Throw exception when DomainClassConverter can't find entity with @PathVariable
body: <p>For instance:</p>
  
  <pre><code>@RestController
  @RequestMapping("/item")
  class ItemController {
  
      @RequestMapping(value = "/{id}", method = GET)
      Item get(@PathVariable("id") Item item) {
          return item;
      }
  }
  </code></pre>
  
  <p>I don't want to handle every time the case which DomainClassConverter returns null when some record doesn't exist. I would like that an exception was thrown automatically. Is it possible?</p>