Update error messages for transfer-schedule commands

- Improve ProcessTransferScheduleAlreadyExistsError message with actionable suggestions
- Update ProcessTransferScheduleNullPairIdError to show different messages:
  - Single transfer with pair_id!=0: shows existing pair_id value
  - Multiple transfers: "Cannot determine which..."

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>