Fix performance issue with unavailable projects.

Instead of just re-throwing the exception, we now log a meaningful message. The previous approach of an uncaught exception caused huge CPU performance issues (up to 100% CPU usage if multiple projects were not accessible).
Note: Don't use uncaught exceptions to control program flow ... :/

src/Soloplan.WhatsON/SerializationHelper.cs

@@ -211,7 +211,8 @@ public async Task<TModel> GetJsonModel<TModel>(string requestUrl, CancellationTo
           throw new OperationCanceledException(ex.Message, ex, token);
         }
 
-        throw;
+        log.Error($"Could not fetch JSON data from {requestUrl}.", ex);
+        return null;
       }
     }