ref #3 Improve message of duplicate model element error

- extract methods
- improve error message
Merge pull request #4 from ChangeVision/3_duplicate_element_message

resolve #3 Improve message of duplicate model element error
Merge remote-tracking branch 'origin/master' into 5_import_property

Conflicts:
	src/main/java/com/change_vision/astah/extension/plugin/reverser/Creator.java