[registrar] Improve error message.

tools/common/StaticRegistrar.cs

@@ -3031,7 +3031,7 @@ namespace XamCore.Registrar {
 				if (type != nativetype) {
 					GenerateConversionToManaged (nativetype, type, setup_call_stack, descriptiveMethodName, ref exceptions, method, $"p{i}", $"arg_ptrs [{i}]", $"mono_class_from_mono_type (xamarin_get_parameter_type (managed_method, {i}))");
 					if (isRef || isOut)
-						throw new Exception ();
+						throw ErrorHelper.CreateError (4163, $"Internal error in the registrar (BindAs parameters can't be ref/out: {descriptiveMethodName}). Please file a bug report at https://bugzilla.xamarin.com");
 					continue;
 				} else if (isRef) {
 					type = type.GetElementType ();