Simplify error message and use nameof for property names

Co-authored-by: javiercn <[email protected]>

Author: Copilot

src/Components/Components/src/Routing/Router.cs

@@ -147,7 +147,7 @@ public async Task SetParametersAsync(ParameterView parameters)
 #pragma warning disable CS0618 // Type or member is obsolete
             if (NotFound != null)
             {
-                throw new InvalidOperationException("Both NotFound and NotFoundPage parameters are set on Router component. NotFoundPage is preferred and NotFound will be deprecated. Consider using only NotFoundPage.");
+                throw new InvalidOperationException($"Setting {nameof(NotFound)} and {nameof(NotFoundPage)} properties simultaneously is not supported. Use either {nameof(NotFound)} or {nameof(NotFoundPage)}.");
             }
 #pragma warning restore CS0618 // Type or member is obsolete
             if (!typeof(IComponent).IsAssignableFrom(NotFoundPage))

src/Components/Components/test/Routing/RouterTest.cs

@@ -297,8 +297,8 @@ public async Task ThrowsExceptionWhenBothNotFoundAndNotFoundPageAreSet()
             await renderer.Dispatcher.InvokeAsync(() =>
                 router.SetParametersAsync(ParameterView.FromDictionary(parameters))));
 
-        Assert.Contains("Both NotFound and NotFoundPage parameters are set on Router component", exception.Message);
-        Assert.Contains("NotFoundPage is preferred and NotFound will be deprecated", exception.Message);
+        Assert.Contains("Setting NotFound and NotFoundPage properties simultaneously is not supported", exception.Message);
+        Assert.Contains("Use either NotFound or NotFoundPage", exception.Message);
     }
 
     internal class TestNavigationManager : NavigationManager