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]

Merging this PR will not alter performance

✅ 313 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing cakebaker:nl_multiple_files_with_missing (f22c43e) with main (2e04477)²}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (852af48) during the generation of this report, so 2e04477 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩