core: Use proper message when scheduling fails with not enough cores

Leftover from scheduling reporting refactoring, the CPUPolicyUnit
still used the old error message style.

This updates it to the new mechanism.

Change-Id: Ibf9247ecded75bb4c4a3bd8e8a7bd23e11ac670b
Bug-Url: https://bugzilla.redhat.com/1129542
Signed-off-by: Martin Sivak <msivak@redhat.com>