Added sanity check to RegisterMvcControllers.

In case the user forgot to call services.AddMvc or services.AddMvcCore,
an expressive exception will be thrown when the user calls
RegisterMvcControllers, since it requires the ASP.NET MVC infrastructure
to be set up. Related to #391.

src/SimpleInjector.Integration.AspNetCore.Mvc/SimpleInjectorAspNetCoreMvcIntegrationExtensions.cs

@@ -57,7 +57,21 @@ public static class SimpleInjectorAspNetCoreMvcIntegrationExtensions
             Requires.IsNotNull(container, nameof(container));
             Requires.IsNotNull(applicationBuilder, nameof(applicationBuilder));
 
-            var manager = applicationBuilder.ApplicationServices.GetRequiredService<ApplicationPartManager>();
+            var manager = applicationBuilder.ApplicationServices.GetService<ApplicationPartManager>();
+
+            if (manager == null)
+            {
+                throw new InvalidOperationException(
+                    string.Format(
+                        CultureInfo.InvariantCulture,
+                        "A registration for the {0} is missing from the ASP.NET Core configuration " +
+                        "system. This is most likely caused by a missing call to services.AddMvcCore() or " +
+                        "services.AddMvc() as part of the ConfigureServices(IServiceCollection) method of " +
+                        "the Startup class. A call to one of those methods will ensure the registration " +
+                        "of the {1}.",
+                        typeof(ApplicationPartManager).FullName,
+                        typeof(ApplicationPartManager).Name));
+            }
 
             var feature = new ControllerFeature();
             manager.PopulateFeature(feature);