nohup: report command execution errors

[wtcpython]

Fixes #12693.

nohup used the error returned by exec only to select the exit status, without reporting why the command could not be started.

Display the underlying execution error before returning, matching GNU's diagnostic while preserving the existing exit codes. strip_errno removes Rust's platform-specific (os error N) suffix.

Extend the existing command-not-found test to check the diagnostic.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/io-errors (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/retry (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)

[wtcpython]

Hi, just a gentle ping in case this got buried. When you have a chance, could you please take a look? Thanks!

[sylvestre]

Please be patient. We have a long backlog

[wtcpython]

Understood — apologies for the premature ping, and thank you for taking the time to review and merge it.