Better exception handling in preprocessor

[ehsk]

Addresses #92

This tiny PR (only preprocess.py):

• Throws exception when batch is empty and sample sequence length does not match the config.
• More information added for debugging when an error is raised in populate_rl_data.

[rafapi]

All these logger calls are heavier than they look, you could keep this lighter and simpler with something like:

    except Exception:
        logger.exception(
            "Error in populate_rl_data",
            extra={
                "Seq length": seq_length,
                "eos_token_id": int(eos_id),
                ...
            },
        )
        raise  <-- this (no e) already keeps the stack trace