Improve handling of non-unicode C strings

[ghost]

This PR is supposed to be merged after #33.

Currently this code results in a panic on Linux:

use leveldb::{database::Database, options::Options};
use std::{ffi::OsStr, os::unix::ffi::OsStrExt, path::Path};

fn main() {
    let _ = Database::<i32>::open(Path::new(OsStr::from_bytes(b"/\xc3\x28")), Options::new())
        .map_err(|e| println!("{:?}", e));
}

With this PR it doesn't panic, but prints

Error { message: "The error returned by LevelDB is not valid UTF-8" }