Add error handling for methods of DeviceModelClient

A response of grpc calls within methods of DeviceModelClient is nil as
soon as an error occured within the call. In some methods values of the
response were directly accessed and returned. This can lead to a
segmentation fault if the response is nil.

Catching errors should prevent this.

controller/plugin/shared/client.go

@@ -27,6 +27,7 @@ func (m *DeviceModelClient) Unmarshal(json []byte, path *gpb.Path) error {
 		Json: json,
 		Path: path,
 	})
+
 	return err
 }
 
@@ -38,6 +39,7 @@ func (m *DeviceModelClient) SetNode(path *gpb.Path, value *gpb.TypedValue) error
 		Path:  path,
 		Value: value,
 	})
+
 	return err
 }
 
@@ -50,7 +52,11 @@ func (m *DeviceModelClient) GetNode(path *gpb.Path, requestForIntendedState bool
 		Path:                    path,
 		RequestForIntendedState: requestForIntendedState,
 	})
-	return resp.GetNodes(), err
+	if err != nil {
+		return nil, err
+	}
+
+	return resp.GetNodes(), nil
 }
 
 // DeleteNode calls the DeleteNode method of the DeviceModelClients client. A
@@ -60,6 +66,7 @@ func (m *DeviceModelClient) DeleteNode(path *gpb.Path) error {
 	_, err := m.client.DeleteNode(context.Background(), &pb.DeleteNodeRequest{
 		Path: path,
 	})
+
 	return err
 }
 
@@ -71,7 +78,11 @@ func (m *DeviceModelClient) Model(filterReadOnly bool) ([]byte, error) {
 	resp, err := m.client.Model(context.Background(), &pb.ModelRequest{
 		FilterReadOnly: filterReadOnly,
 	})
-	return resp.Json, err
+	if err != nil {
+		return nil, err
+	}
+
+	return resp.Json, nil
 }
 
 // Diff calls the Diff method of the DeviceModelClients client. A request is
@@ -82,7 +93,11 @@ func (m *DeviceModelClient) Diff(original, modified []byte) (*gpb.Notification,
 		Original: original,
 		Modified: modified,
 	})
-	return resp.GetNotification(), err
+	if err != nil {
+		return nil, err
+	}
+
+	return resp.GetNotification(), nil
 }
 
 // ValidateChange calls the ValidateChange method of the DeviceModelClients
@@ -95,7 +110,11 @@ func (m *DeviceModelClient) ValidateChange(operation mnepb.ApiOperation, path *g
 		Path:      path,
 		Value:     value,
 	})
-	return resp.GetModel(), err
+	if err != nil {
+		return nil, err
+	}
+
+	return resp.GetModel(), nil
 }
 
 // PruneConfigFalse calls the PruneConfigFalse method of the DeviceModelClients
@@ -106,6 +125,10 @@ func (m *DeviceModelClient) PruneConfigFalse(value []byte) ([]byte, error) {
 	resp, err := m.client.PruneConfigFalse(context.Background(), &pb.PruneConfigFalseRequest{
 		Value: value,
 	})
+	if err != nil {
+		return nil, err
+	}
+
 	return resp.GetModel(), err
 }