fix: catch OSError when Cargo.toml is missing during --cargo introspection

[DhruvTilva]

Small, isolated fix: catches a missing-file error that currently crashes --cargo with a raw traceback instead of a clean message. One-line change, no new behavior, no risk to existing functionality.

Before: --cargo on a missing Cargo.toml crashes with a raw 10-line Python traceback — confusing for anyone who just typo'd a path.

After: Same situation now exits with one clean line: error: [Errno 2] No such file or directory: '...\Cargo.toml' — and exit code 1, so scripts/CI calling this still detect failure correctly.

Problem

graphify extract <path> --cargo against a folder with no Cargo.toml throws a raw, unhandled FileNotFoundError straight at the user:
FileNotFoundError: [Errno 2] No such file or directory: 'C:\path\Cargo.toml'
A typo'd path or running --cargo one directory too high gets you a 10-line traceback instead of a sentence telling you what went wrong.

Root cause

introspect_cargo() opens Cargo.toml directly, so a missing file raises FileNotFoundError — a subclass of OSError. The call site in __main__.py only catches ConnectionError and ImportError, so this one slips through uncaught.

Testing

• Missing Cargo.toml → error: [Errno 2] No such file or directory: '...\Cargo.toml', exit code 1, no traceback
• Valid Cargo.toml → introspection still runs and extracts crate nodes exactly as before
• pytest, ruff check, pyright all green on the changed file

[safishamsi]

Merged into v8 as aa06e10 (cherry-picked onto current v8). Full suite green on 3.10 + 3.12. Thanks @DhruvTilva!

[DhruvTilva]

Merged into v8 as aa06e10 (cherry-picked onto current v8). Full suite green on 3.10 + 3.12. Thanks @DhruvTilva!

Great @safishamsi
Glad it helps :)