NotFound result if key is not found

Author: YakupIpek

src/Stratis.Bitcoin.Features.SmartContracts/ReflectionExecutor/Controllers/SmartContractsController.cs

@@ -190,7 +190,7 @@ public IActionResult GetStorage([FromQuery] GetStorageRequest request)
 
             if (storageValue == null)
             {
-                return this.Json(new
+                return this.NotFound(new
                 {
                     Message = string.Format("No data at storage with key {0}", request.StorageKey)
                 });