refactor(core): Improve domain processing and error handling

Changes:
- Update process_domains function for better reliability
- Add file existence checks in key functions
- Implement comprehensive error handling

Technical details:
- Add checks in apply_whitelist and load_lists
- Improve error messages and exit handling
- Fix ShellCheck compliance issues
- Add proper input validation

Makes script more robust by preventing file-related errors and improving error handling.

Author: SMKRV