Improve validation and error messages for large inputs

[carpalsgrabby]

Summary

When extremely large --leaves or --arity values are passed (e.g. > 10^15), Python can overflow or become extremely slow in exponentiation. The script should handle this more gracefully.

Proposal

• Add upper bounds (or at least warnings).
• Catch OverflowError on exponentiation.
• Provide user-friendly error messages.

Benefit

Prevents crashes and guides users when input is unrealistic.