engine: Improve reporting when scheduling fails on network

This fixes and improves the way we construct canDoAction messages
in the NetworkPolicyUnit.

Change-Id: Ic60daf20842ba69457b293f451060e21381faf80
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1132951
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1114987
Signed-off-by: Martin Sivak <msivak@redhat.com>