Improve check_migration command error message
Improve check_migration command error message

This change adds some details (expected heads) to the error raised by
neutron-db-manage check_migration command in order to help debugging.

Change-Id: Icfc6fc5722b5a3b7c4a36131553eeba0941d12e3

adelina-t/neutron,mmnelemane/neutron,eonpatapon/neutron,neoareslinux/neutron,glove747/liberty-neutron,chitr/neutron,yanheven/neutron,shahbazn/neutron,adelina-t/neutron,skyddv/neutron,skyddv/neutron,sebrandon1/neutron,watonyweng/neutron,watonyweng/neutron,takeshineshiro/neutron,mahak/neutron,barnsnake351/neutron,openstack/neutron,dims/neutron,eonpatapon/neutron,jacknjzhou/neutron,dims/neutron,cisco-openstack/neutron,paninetworks/neutron,swdream/neutron,wolverineav/neutron,mattt416/neutron,wenhuizhang/neutron,shahbazn/neutron,javaos74/neutron,barnsnake351/neutron,swdream/neutron,yanheven/neutron,vivekanand1101/neutron,klmitch/neutron,cisco-openstack/neutron,dhanunjaya/neutron,MaximNevrov/neutron,huntxu/neutron,SamYaple/neutron,openstack/neutron,bigswitch/neutron,neoareslinux/neutron,silenci/neutron,asgard-lab/neutron,javaos74/neutron,mahak/neutron,JianyuWang/neutron,mattt416/neutron,igor-toga/local-snat,sebrandon1/neutron,mmnelemane/neutron,wolverineav/neutron,wenhuizhang/neutron,vivekanand1101/neutron,apporc/neutron,noironetworks/neutron,suneeth51/neutron,silenci/neutron,jumpojoy/neutron,bigswitch/neutron,MaximNevrov/neutron,eayunstack/neutron,paninetworks/neutron,sasukeh/neutron,igor-toga/local-snat,chitr/neutron,suneeth51/neutron,jumpojoy/neutron,dhanunjaya/neutron,JianyuWang/neutron,cloudbase/neutron,asgard-lab/neutron,apporc/neutron,glove747/liberty-neutron,noironetworks/neutron,openstack/neutron,SamYaple/neutron,takeshineshiro/neutron,jacknjzhou/neutron,cloudbase/neutron,huntxu/neutron,mahak/neutron,sasukeh/neutron,klmitch/neutron,eayunstack/neutron