fix: do not check/verify slice units if recursive errors are to be ignored

fix: do not check/verify slice units if recursive errors are to be ignored

Before this fix, when recursive-errors was set to 'no' during a systemd-analyze
verification, the parent slice was checked regardless. The 'no' setting means that,
only the specified unit should be looked at and verified and errors in the slices should be
ignored. This commit fixes that issue.

Example:

Say we have a sample.service file:

[Unit]
Description=Sample Service

[Service]
ExecStart=/bin/echo "a"
Slice=support.slice

Before Change:

systemd-analyze verify --recursive-errors=no maanya/sample.service
Assertion 'u' failed at src/core/unit.c:153, function unit_has_name(). Aborting.
Aborted (core dumped)

After Change:
systemd-analyze verify --recursive-errors=no maanya/sample.service
{No errors}