pr: bad error message when directory doesn't exist

GNU pr:
```
$ pr qwe
pr: qwe: No such file or directory
```
uutils pr:
```
$ cargo run pr qwe
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.12s
     Running `target/debug/coreutils pr qwe`
pr: No such file or directory (os error 2)
```
directory name is omitted from uutils pr error, error code should be stripped with strip_errno()