cat: always exit with 1 on error

[mtvb]

I am unsure if returning the number of errors as exit code is a feature, and found the commit that introduced it.
2106ddd08 ("migrate cat code to UResult #2464")
Before it, cat_files returned Err(error_count: u32) and uumain collapsed any error into a plain exit code 1.

My commit restores this behavior, matching gnu cat.

I had to modify one test, "test_write_to_self", it was expecting exit code 2, now it will return 1.

Fixes #14215

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/dd/misc. tests/dd/misc is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/df/over-mount-device. tests/df/over-mount-device is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/follow-name (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cat/splice is no longer failing!
Congrats! The gnu test tests/cp/cp-a-selinux is no longer failing!
Congrats! The gnu test tests/cut/cut is no longer failing!
Congrats! The gnu test tests/cut/mb-non-utf8 is no longer failing!
Congrats! The gnu test tests/dd/partial-write is no longer failing!
Congrats! The gnu test tests/expand/mb is no longer failing!
Congrats! The gnu test tests/ls/stat-free-symlinks is no longer failing!
Congrats! The gnu test tests/misc/close-stdout is no longer failing!
Congrats! The gnu test tests/mktemp/write-error is no longer failing!
Congrats! The gnu test tests/mv/dir2dir is no longer failing!
Congrats! The gnu test tests/mv/mv-exchange is no longer failing!
Congrats! The gnu test tests/nl/multibyte is no longer failing!
Congrats! The gnu test tests/od/od-float is no longer failing!
Congrats! The gnu test tests/od/od-j is no longer failing!
Congrats! The gnu test tests/ptx/ptx-overrun is no longer failing!
Congrats! The gnu test tests/sort/sort-merge-fdlimit is no longer failing!
Congrats! The gnu test tests/unexpand/mb is no longer failing!
Note: The gnu test tests/dd/fail-ftruncate-fstat was skipped on 'main' but is now failing.