Decode error messages

Currently, the macOS Keyring backend hard-codes a few error codes to messages. Better would be to use the API to [resolve error codes to human-readable message strings](https://developer.apple.com/documentation/security/1394686-seccopyerrormessagestring).

I started looking into how to do it. I imagine it goes something like:

```
>> api._sec.SecCopyErrorMessageString(-25244)
-1797030505
```

But probably something needs to translate that integer response into a `CFString` object and convert that object to a Python string.

Maybe someone would be interested in determining how to do that?