replace ObjectDisposedException with more user friendly one

We received a stacktrace from Zakupki:

```
System.ObjectDisposedException: Доступ к ликвидированному объекту невозможен. 
Имя объекта: "IronSharp.Core.JsonContent". 
в System.Net.Http.HttpContent.ReadAsStringAsync() 
в IronSharp.Core.RestResponseException.AppendDebugInfo(HttpResponseMessage response) в c:\Users\admin\Documents\GitHub\iron_dotnet\src\IronSharp.Core\Exceptions\RestResponseException.cs:строка 45 
в IronSharp.Core.RestResponseException..ctor(String message, HttpResponseMessage response) в c:\Users\admin\Documents\GitHub\iron_dotnet\src\IronSharp.Core\Exceptions\RestResponseException.cs:строка 9 
в IronSharp.IronMQ.QueueClient.Get(Nullable`1 n, Nullable`1 timeout, Nullable`1 wait) в c:\Users\admin\Documents\GitHub\iron_dotnet\src\IronSharp.IronMQ\QueueClient.cs:строка 340 
в IronSharp.IronMQ.QueueClient.Next(Nullable`1 timeout) в c:\Users\admin\Documents\GitHub\iron_dotnet\src\IronSharp.IronMQ\QueueClient.cs:строка 390 
в ByndyuSoft.Zakupki360.Infrastructure.DataAccess.IronMQ.Sources.MqQuery`1.Ask(IQueueCriterion criterion) в c:\Development\zakupki360\Mlg.Spending.Web\sources\Zakupki360.Infrastructure.DataAccess.IronMQ\Sources\MqQuery.cs:строка 29
```

We needn't to throw exception from constructor of our exception. So I surrounded  `request.Content.ReadAsStringAsync()` with try-catch