…ror and add user-facing message

- Change FailureType from config_error to system_error in _process_partitions_with_errors
  and the final aggregated exception in create_and_get_completed_partitions
- Add user-facing message field to both AirbyteTracedException instances
- Update test assertion to expect system_error instead of config_error

The previous config_error classification incorrectly prevented automatic retries
and misled oncall into thinking these were user configuration issues. These are
job execution failures (e.g., expired download URLs, HTTP 403) that are not
caused by user configuration.

Co-Authored-By: bot_apk <apk@cognition.ai>