head: continue processing files even if one has io error

[iburaky2]

Currently head exits with an error when it encounters an I/O error, whereas GNU head continues with the rest of the files provided in the arguments. Closes #11846

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/head/head-write-error. tests/head/head-write-error is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/tail/pid-pipe. tests/tail/pid-pipe is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)

[oech3]

By the way, this does not happend with head . file. We should handle /proc/self/men at the place . is handled.

I found an another issue #11972 .

[iburaky2]

We need to properly distinguish between read and write errors to fix the incorrect error messages, wrapping all std::io::copy errors in stdout error isn't correct.

I tried looking at the ErrorKind and but I'm getting Uncategorized for coreutils head /proc/self/mem, I don't think std::io::copy gives us enough information to properly differentiate read and write errors atm.

Maybe we should break up std::io::copy into seperate read and write steps, or use something from uucore. I haven't had the chance to properly look into uucore yet.

Anyway that is a seperate issue than the one this PR addresses