Create a custom exception class ValidationError that inherits from std::exception, then use it in a function.

Requirements for ValidationError:
1. Private std::string message field
2. Constructor ValidationError(std::string msg): stores the message
3. Override what() const noexcept to return message.c_str()

Also write a function validateUsername(const std::string& name):
- If name is empty, throw ValidationError("Username cannot be empty")
- If name.length() < 3, throw ValidationError("Username too short")
- Otherwise return name

Do NOT write a main function.