Throw a CoapEndpointException instead of InvalidOperationException

src/CoAPNet/CoapClient.cs

@@ -124,7 +124,7 @@ public async Task<CoapReceiveResult> ReceiveAsync(CancellationToken token)
                     : await resultTask;
 
             if (Endpoint == null)
-                throw new InvalidOperationException($"{nameof(CoapClient)} is in an invalid state");
+                throw new CoapEndpointException($"{nameof(CoapClient)} does not have a valid endpoint");
 
             StartReceiveAsyncInternal();