Expose SAIC’s return code and message on exceptions #54

When a call fails, SaicApiException only keeps the combined text, "return code: 8, message: Vehicle not locked.". To act on the reason, callers have to parse that string back apart.

Why it matters: the same code can mean different things, and the message is the only way to tell them apart:

• Code 8 is used both for "vehicle not locked" and for the remote command limit ("start the vehicle with the key"). They need different handling: one asks the user to lock the car, the other to drive it.
• Code 4 means the car couldn't be reached, e.g. asleep. Integrations want to treat that as "unreachable" rather than as a failure.
• 401 / 403 mean the session has expired.

In mg-saic-ha we were parsing str(e) to tell these apart, which breaks if the wording ever changes.

Suggested change:

• keep return_code and SAIC's message (saic_message) as separate attributes on SaicApiException;
• add a SaicReturnCode enum for the codes with a known meaning (0, 4, 6, 8, 401, 403, and the library's own 500 for unexpected failures);
• add simple checks: is_vehicle_unreachable, is_request_rejected, is_vehicle_not_locked, is_logged_out, is_unexpected_failure;
• keep str(e) exactly as it is, so existing code isn't affected.

Part of #26. I'll open a PR for it.