[fix] fix misc not important issues

- correct all icon usage. use 'CANCEL' instead of 'ERROR' to indicate error correctly. and move these icon const into UTILS_icons_manager.py
- auto rename generated rail section to match the default name standard.
- give more clear signal about import and export bm file by using Operator.report() function.

ballance_blender_plugin/UTILS_icons_manager.py

@@ -3,6 +3,10 @@ import bpy.utils.previews
 import os
 from . import UTILS_constants
 
+blender_info_icon = 'INFO'
+blender_warning_icon = 'ERROR'
+blender_error_icon = 'CANCEL'
+
 # ImagePreviewCollection ccreated by Blender
 floor_icons = None
 # a map. key is block name, value is loaded icon id