Assert that AlreadyClosedException message contains the prefix we want

Author: michaelklishin

projects/client/Unit/src/unit/TestExceptionMessages.cs

@@ -67,6 +67,7 @@ public void TestAlreadyClosedExceptionMessage()
             } catch (AlreadyClosedException e)
             {
                 Assert.That(e, Is.TypeOf(typeof(AlreadyClosedException)));
+                Assert.IsTrue(e.Message.StartsWith("Already closed"));
             }
         }
     }