du: don't panic if file doesn't exist

[ajanicij]

This PR solves a problem that file opening error causes panic in du.

Technical details

Running du with a nonexistent file causes a panic:

> du -X /proc/self/mem

thread 'main' (653) panicked at src/uu/du/src/du.rs:763:20:
Could not parse line: Os { code: 5, kind: Uncategorized, message: "Input/output error" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fish: Job 1, 'du -X /proc/self/mem' terminated by signal SIGABRT (Abort)

The problem is that line

let file = File::open(filename).expect("no such file");

causes a panic. (Also, the message no such file is incorrect; File::open can fail for other reasons; for example,
if we pass a directory name instead of a file.)

[oech3]

Sorry. But #11996

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/unexpand/bounded-memory is now passing!
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[ajanicij]

:-)
I was too slow. 11996 was there first. Closing this PR.