Fix serialization tests & don't block thread (#846)

* Fix serialization tests & don't block thread
- Fixed potential null ref when closing streams
- Always clean up serialization queue if an error occurs
- Stop blocking dispatcher thread by not awaiting task that processes the message
- Improved error logging in EventFlowValidator to help debug issues
- Close stream on exception

src/Microsoft.SqlTools.Hosting/Hosting/Contracts/Error.cs

@@ -20,5 +20,10 @@ namespace Microsoft.SqlTools.Hosting.Contracts
         /// Error message
         /// </summary>
         public string Message { get; set; }
+
+        public override string ToString()
+        {
+            return $"Error(Code={Code},Message='{Message}')";
+        }
     }
 }