Add ProcError::Incomplete

[dalance]

Rarely from_reader seems to be panic.
For example, the following code becomes panic once every 30 minutes - 1 hour.

fn main() {
    loop {
        std::panic::catch_unwind(|| {
            for p in procfs::all_processes() {
                p.io();
                p.status();
                p.maps();
                p.mountstats();
            }
        });
    }
}

Panic occurs at Stat::from_reader, Status::from_reader and Io::from_reader.
(Process::maps and MountStat::from_reader are no problem)

This is because that opening procfs file is success but reading the contents is failed.
Usually contents seems to be empty, but sometimes incomplete contents is read.

To handle this error, I added ProcError::Incomplete and removed unwrap in the code of parsing contents.

[dalance]

After 8 hours execution, Process::maps becomes panic.
I think MountStat::from_reader should be fixed too.

[eminence]

Sorry for taking so long to review this. And thanks a lot for this change (and all of the others you have made) -- it's really helped to improve the procfs library.

This sounds good to me. I'm going to test it (via your small test loop) on my personal computers, as well as some computers at work which are heavily loaded and see lots of process churn. In the meantime, I'll merge this.

[dalance]

Thanks