cmp: use .map_err

[oech3]

No description provided.

[codspeed-hq]

Merging this PR will degrade performance by 4.77%

❌ 1 regressed benchmark
✅ 25 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	cmp_parser	5.6 µs	5.9 µs	-4.77%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing oech3:cmp-map_err (7c23ada) with main (d266f9b)}

[github-actions]

GNU diffutils testsuite comparison:

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

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

New test failures (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

[marc-hb]

Need more coffee? Sincere apologies for the noise.

[oech3]

No problem. Thankyou for reviewing.