utils/error library

1. skin_assets_code_gen package
2. documentation
3. utils/error.dart

skin_assets_code_gen package

1. Libraries
2. asset_generator
3. flutter_generator
4. pubspec
5. settings/config
6. settings/config_default
7. settings/flavored_asset
8. utils/cast
9. utils/color
10. utils/error
11. utils/file
12. utils/identifer
13. utils/map
14. utils/res_utils
15. utils/string