Azure: Display proper error message on failure to delete

If there are other resources in the resource group that are in use
by resources in other groups then the deletion will be blocked.
The error message displayed is not user friendly and hence adding
a check to see if the error code returned by azure autorest is
409, which is ResourceGroupDeletionBlocked, and display a proper
message to the user.

pkg/destroy/azure/azure.go

@@ -160,6 +160,9 @@ func (o *ClusterUninstaller) Run() error {
 				if isAuthError(err) {
 					cancel()
 					errs = append(errs, errors.Wrap(err, "unable to authenticate when deleting resource group"))
+				} else if isResourceGroupBlockedError(err) {
+					cancel()
+					errs = append(errs, errors.Wrap(err, "unable to delete resource group, resources in the group are in use by others"))
 				}
 				return
 			}
@@ -505,6 +508,24 @@ func isAuthError(err error) bool {
 	return false
 }
 
+func isResourceGroupBlockedError(err error) bool {
+	if err == nil {
+		return false
+	}
+
+	var dErr autorest.DetailedError
+	if errors.As(err, &dErr) {
+		switch statusCode := dErr.StatusCode.(type) {
+		case int:
+			if statusCode == 409 {
+				return true
+			}
+		}
+	}
+
+	return false
+}
+
 func deleteApplicationRegistrations(ctx context.Context, appClient graphrbac.ApplicationsClient, spClient graphrbac.ServicePrincipalsClient, logger logrus.FieldLogger, infraID string) error {
 	errorList := []error{}