head: fix skipping other files when getting errors from reading

[lauchimoon]

Fixes #13127.

The issue was that head_file was returning early in case of an error, making uu_head exit early and ignore the rest of files to be processed.

The fix catches the error given by head_file, prints it, and keeps iterating over each file.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/tty-eof (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/tail/pipe-f is now being skipped but was previously passing.
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[cakebaker]

Can you please add a test to tests/by_util/test_head.rs to ensure we don't regress in the future? Thanks.

[lauchimoon]

Hello @cakebaker, I added the test checking against /dev/video0. Let me know if it's good.