sum: extract error message from OS

[oech3]

No description provided.

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 149.10 MB (+104 KB, +0.07%)

Significant per-binary changes:
  [           1.10 MB ->    1.18 MB  (+76 KB, +6.74%)
  mkfifo      1.09 MB ->    1.16 MB  (+76 KB, +6.81%)
  test        1.10 MB ->    1.18 MB  (+76 KB, +6.74%)
  install     1.25 MB ->    1.32 MB  (+72 KB, +5.62%)
  mkdir       1.10 MB ->    1.17 MB  (+72 KB, +6.38%)
  mknod       1.10 MB ->    1.17 MB  (+72 KB, +6.41%)
  chmod       1.15 MB ->    1.22 MB  (+68 KB, +5.76%)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)