improve error handling

[lovasoa]

fixes #115

See sqlpage/SQLPage#814

[nagisa]

I think it'd be fine to return None from guess_os_stack when procfs isn't available, but in principle it also doesn't change that for most intents and purposes systems configured that way are sorta broken for the purposes of *-linux-gnu targets at least.

[lovasoa]

@nagisa I reorganized guess_os_stack to have separate files per backend, addressed your comments, and added more error handling where it was missing on other platforms.

[lovasoa]

Great, thank you very much @nagisa !