Improve exception handling in UserService

[stevenkiwanukakafeero]

UserService currently uses generic RuntimeException for error handling.

Problem:
Generic exceptions make debugging difficult and reduce code clarity.

Solution:
Introduce specific exceptions like UserNotFoundException and DuplicateUserException.

Importance:
Improves maintainability, debugging, and API clarity.

[RonaldRonnie]

Great catch, @stevenkiwanukakafeero. Replacing generic RuntimeExceptions with specific domain exceptions (like UserNotFoundException and DuplicateUserException) is a massive win for maintainability.

Feel free to assign this to yourself or open a PR when ready, and I'll gladly review it!