[townsmcp]

Closes #54. Part of #26.

SaicApiException now keeps SAIC's reply as separate values, so callers don't have to parse str(e) to find out why a call failed.

Changes

• return_code and saic_message: new attributes on SaicApiException and all its subclasses. return_code is SAIC's numeric code (or None); saic_message is the message on its own.
• SaicReturnCode: a new enum of the codes with a known meaning:

Code	Name	Meaning
0	SUCCESS
4	VEHICLE_UNREACHABLE	the car can't be reached, e.g. asleep
6	SERVICE_UNAVAILABLE
8	REQUEST_REJECTED	refused, for several reasons: read the message
401 / 403	UNAUTHORIZED / FORBIDDEN	session no longer valid
500	UNEXPECTED_FAILURE	the library's own code for an unexpected failure

Codes not listed still come through unchanged in return_code.

• Checks for the common cases: is_vehicle_unreachable, is_request_rejected, is_vehicle_not_locked (code 8 with "not locked" in the message, as opposed to the remote command limit, which is also code 8), is_logged_out (always true on SaicLogoutException) and is_unexpected_failure.
• Exceptions now call super().__init__(), so e.args is set and they pickle cleanly.
• str(e) is unchanged ("return code: <code>, message: <message>", or just the message), including SaicApiRetryException's event_id suffix. Existing code that reads the text keeps working.

Tests

tests/test_exceptions.py covers:

• the unchanged text, args and pickling;
• each check across codes 4, 6, 8 (both "not locked" and the command limit), 401, 403 and no code;
• the retry and logout exceptions;
• two cases through the real response handling: a code-8 "Vehicle not locked." reply, and a code-4 reply during event polling.

Notes

• README: a short "Errors" section, added at the end.
• No dependencies: this PR is independent of the seat and window PRs and merges cleanly alongside them.
• Checks: ruff (check and format), mypy, pylint and pytest all pass locally on Python 3.13.