Error message for missing users and roles in linkahead_admin.py

Summary

Added error messages when retrieving and updating an user or role with linkahead_admin. Addresses #200 (closed)

Focus

The code changes are in the second diff version, the other changes are exclusively indentation fixes for docstrings. Would it be useful to also add success messages for operations that do not print anything?

Check List for the Author

• All automated tests pass
• Reference related issues
• Not necessary: Appropriate user and developer documentation for new code
• Annotations in code

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Appropriate user and developer documentation for new code
• The intended behavior is reproducible