Improve ERROR_ON_CLONE message
Improve ERROR_ON_CLONE message

In case ERROR_ON_CLONE is true and triggers a failure for a missing
project, suggest a remedial action.  On their own, people have
come up with remedies that include altering the value of ERROR_ON_CLONE
which rather defeats the purpose.

Change-Id: I28d7f2c184f8440b774fefaa8ec7002d6708db95

richliu/devstack,pombredanne/devstack,parul24/devstack,dirkmueller/devstack,Mahito/devstack,varunarya10/devstack,samgoon/devstack,openstack-dev/devstack,avvocatodemarchis/devstack,olivierlemasle/devstack,thomasem/devstack,avvocatodemarchis/devstack,group-policy/devstack,dtroyer/devstack,prithivm/branch01,prithivm/branch01,bigswitch/devstack,dirkmueller/devstack,ozamiatin/devstack,bswartz/devstack,bigswitch/devstack,parul24/devstack,thomasem/devstack,costingalan/devstack,mssumanth/devstack,openstack-dev/devstack,samgoon/devstack,noironetworks/devstack,pombredanne/devstack,richliu/devstack,dtroyer/devstack,mssumanth/devstack,ozamiatin/devstack,bswartz/devstack,nawawi/openstack,Mahito/devstack,noironetworks/devstack,costingalan/devstack,varunarya10/devstack,olivierlemasle/devstack,nawawi/openstack,group-policy/devstack