Update Block a user form (#37359)

Use the new "form-fetch-action" for better user experience, and use
JSONError to show error messages.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/user/block.go

@@ -90,7 +90,7 @@ func GetBlocking(ctx context.Context, blockerID, blockeeID int64) (*Blocking, er
 		return nil, err
 	}
 	if len(blocks) == 0 {
-		return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
+		return nil, util.NewNotExistErrorf("blocking record doesn't exist")
 	}
 	return blocks[0], nil
 }