title: How to return 404 with Spring WebFlux
body: <p>I'm having a controller like this one (in Kotlin):</p>
  
  <pre><code>@RestController
  @RequestMapping("/")
  class CustomerController (private val service: CustomerService) {
      @GetMapping("/{id}")
      fun findById(@PathVariable id: String,
                   @RequestHeader(value = IF_NONE_MATCH) versionHeader: String?): Mono&lt;HttpEntity&lt;KundeResource&gt;&gt; =
          return service.findById(id)
              .switchIfEmpty(Mono.error(NotFoundException()))
              .map {
                  // ETag stuff ...
                  ok().eTag("...").body(...)
              }
  }
  </code></pre>
  
  <p>Im wondering whether there is a better approach than throwing an exception which is annotated with <code>@ResponseStatus(code = NOT_FOUND)</code></p>