nl: continue if a file can't be opened

[cakebaker]

Currently, we stop processing the specified files if one can't be opened whereas GNU nl keeps processing.

This PR fixes the issue. It should make the multiple-files.sh GNU test pass.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cp/sparse-to-pipe is no longer failing!
Congrats! The gnu test tests/nl/multiple-files is no longer failing!
Congrats! The gnu test tests/nl/nl is no longer failing!
Congrats! The gnu test tests/tail/pipe-f2 is no longer failing!
Congrats! The gnu test tests/basenc/bounded-memory is now passing!
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[codspeed-hq]

Unable to generate the performance report

There was an internal error while processing the run's data. We're working on fixing the issue. Feel free to contact us on Discord or at [email protected] if the issue persists.