fix: raise clear error when required SSH connection params are None

[prabindersinghh]

Summary

Add validation for required SSH connection parameters — previously
crashed with confusing TypeError if any were None.

Context / Motivation

If a user forgets to set ssh_key_file, username, or address via
@slurm(...) or environment variables, the code crashed with:
TypeError: argument should be str or an os.PathLike object, not NoneType

This gives no indication of what went wrong or how to fix it.

Changes Made

• Added validation in __init__ before first parameter use
• Raises SlurmException with clear message for each missing param
• Example: "ssh_key_file is required. Set it via @slurm(ssh_key_file=...)
  or the METAFLOW_SLURM_SSH_KEY_FILE environment variable."

Testing

Manually verified: instantiating SlurmClient with None values now
raises SlurmException with correct message instead of TypeError.