Implement FwMgmt service error handling

[dymk]

Task\n\nImplement FwMgmt service error handling and process_indirect (return NotSupported). Add unit tests for FwMgmt service and MessageHandler dispatch.\n\n## Files\n- ec-service-lib/src/services/fw_mgmt.rs\n- ec-service-lib/src/message_handler.rs\n\n## PR\n- #58\n\n## Parent\nPart of #60