Add more information to assertion message

Author: drewnoakes

src/NetMQ/Utils/Assumes.cs

@@ -9,7 +9,7 @@ internal static class Assumes
         [Conditional("DEBUG")]
         public static void NotNull<T>([NotNull] T o) where T : class?
         {
-            Debug.Assert(o is object);
+            Debug.Assert(o is object, $"Unexpected null of type {typeof(T).Name}");
         }
 #pragma warning restore CS8777 // Parameter must have a non-null value when exiting.
     }