fix(backend): type is wrong for error

This was not an issue because the testing code did not check it. It is fixed now.

Author: Liu233w

backend/src/AcmStatisticsBackend.Core/ServiceClients/CrawlerApiBackendClient.cs

@@ -37,7 +37,7 @@ public async Task<IReadOnlyCollection<CrawlerMetaItem>> GetCrawlerMeta()
 #pragma warning disable SA1300
         private class GetMetaReturn
         {
-            public string error { get; set; }
+            public bool error { get; set; }
 
             public IDictionary<string, DataItem> data { get; set; }
         }