cmp.rs: simplify by .ok_or

[oech3]

No description provided.

[github-actions]

GNU diffutils testsuite comparison:

Test results comparison:
  Current:   TOTAL: 33 / PASSED: 8 / FAILED: 21 / SKIPPED: 4
  Reference: TOTAL: 33 / PASSED: 0 / FAILED: 33 / SKIPPED: 0

Changes from main branch:
  TOTAL: +0
  PASSED: +8
  FAILED: -12

Test improvements (12):
  + basic
  + bignum
  + brief-vs-stat-zero-kernel-lies
  + bug-64316
  + cmp
  + diff3
  + help-version
  + large-subopt
  + strcoll-0-names
  + strip-trailing-cr
  + timezone
  + y2038-vs-32bit

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (1a8d7f9) to head (b553e03).

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #243   +/-   ##
===========================
===========================

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[cakebaker]

Thanks!