Add 404 status to @ApiResponses

Author: bernd

src/main/java/org/graylog/plugins/usagestatistics/UsageStatsOptOutResource.java

@@ -56,6 +56,7 @@ public UsageStatsOptOutResource(UsageStatsOptOutService usageStatsOptOutService)
     @Timed
     @ApiOperation(value = "Get opt-out status")
     @ApiResponses(value = {
+            @ApiResponse(code = 404, message = "Opt-out status does not exist"),
             @ApiResponse(code = 500, message = "Internal Server Error")
     })
     public UsageStatsOptOutState getOptOutState() {