[Reliability] Improved Error Parsing for Subprocess Training

[DsThakurRawat]

Description

The LerobotBackend invokes lerobot-train via a subprocess and currently only reports the exit code on failure.

Rationale

Robot training often fails due to hardware-specific issues (OOM, CUDA errors, disk full). Swallowing these behind a generic 'exited with code 1' message makes debugging difficult for users.

Tasks

• Implement a regex-based error parser for the subprocess stderr stream.
• Map common strings (e.g., 'OutOfMemoryError', 'CUDA error: out of memory') to actionable remediation hints.
• Surface these hints in the FinetuneResult.error payload.