Update requirements file and added error messages for gNMI OpenConfig collector

- Added pyang and pyangbind to requirements.in
- Created ErrorMessages.py for standardized error handling
- Adjusted BASE_PATH in clone-yang-models.sh for correct directory structure
- Updated YangHandler.py to include openconfig-ospf-types
- Refactored imports in __init__.py for better readability
- Added test utilities in messages_gnmi_openconfig.py for KPI packet creation
- Modified test_gnmi_openconfig_collector.py to utilize new test utilities