Narrow optional import exception handling in pyisolate.__init__

[seanwevans]

Motivation

• Avoid silently swallowing unrelated import-time errors in optional modules so developer-visible bugs surface during import.
• Continue to treat the checkpoint/migration features as optional when the cryptography dependency is absent.
• Preserve the existing public API and error semantics for users when optional crypto support is unavailable.

Description

• Replace broad except Exception around from .checkpoint import checkpoint, restore and from .migration import migrate with except (ModuleNotFoundError, ImportError) and inspect the exception before deciding to swallow it.
• Re-raise ModuleNotFoundError/ImportError instances that do not indicate the cryptography dependency is missing so unexpected import-time failures propagate.
• Preserve the fallback callables checkpoint, restore, and migrate that raise ModuleNotFoundError("cryptography is required ...") to keep the same API shape and messages.
• Add brief inline comments explaining that only dependency-related import failures are intentionally trapped.

Testing

• Ran python -m compileall pyisolate/__init__.py which completed successfully.

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Codex Task