Add check for tag existence before tagging local Git repository

- Raise Stop if tag exists instead of failing during `git tag` call
- Add failing test in TestTagRelease with pre-created tag
Add check for tag validity before tagging local Git repository

- Raise Stop if tag reference name is invalid instead of failing during `git tag` call
- Add failing test in TestTagRelease with invalid tag (ending with '..')
merge master -D org -I include -E exclude: PR 89 (Tag release)