fix: restore Unwrap on the error returned by Exit

[fadiroot]

What type of PR is this?

• bug

What this PR does / why we need it:

Exit() wraps the caller's error in an unexported exitError. Since #1545 (issue #1090) that wrapper had an Unwrap() method so errors.Is / errors.As could reach the original error through the ExitCoder. The method was dropped in 05071ea ("Add error tests") without being mentioned in the commit, so on v3 today:

var ErrNotFound = errors.New("not found")
err := cli.Exit(ErrNotFound, 2)
errors.Is(err, ErrNotFound) // false on main; true on v2 and on v3 before Oct 2024

origin/v2-maint still has the method.

• errors.go: add func (ee *exitError) Unwrap() error back.
• errors_test.go: TestExit_Unwrap checks errors.Is, errors.Unwrap, errors.As to ExitCoder, and the non-error message form. It fails on main and passes with this change.

No exported API changes (exitError is unexported), so godoc-current.txt is unaffected.

Release Notes

Restore `Unwrap()` on the error returned by `Exit()`, so `errors.Is` and `errors.As` see the wrapped error again.