MR !803 Thread #2: Add PyError exception hierarchy for better error handling

Add PyError as the base exception for all PY module errors and
PyValidationError as the base for validation-related errors.
This allows users to catch all PY errors or just validation errors
with a single except clause.

Addresses: IMP-4 from code review

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