Flavor: correct confusing error message about flavorRef
Flavor: correct confusing error message about flavorRef

There was a try/except block that was not used anymore since the
schema check catches invalid input. Raise a FlavorNotFound exception
when an invalid flavor is provided.

Change-Id: I76be8942d59b408c7512658968cdb12733772ead

alaski/nova,Juniper/nova,cloudbase/nova,phenoxim/nova,phenoxim/nova,alaski/nova,vmturbo/nova,rajalokan/nova,rahulunair/nova,jianghuaw/nova,mikalstill/nova,Juniper/nova,rahulunair/nova,vmturbo/nova,klmitch/nova,vmturbo/nova,sebrandon1/nova,Juniper/nova,jianghuaw/nova,sebrandon1/nova,gooddata/openstack-nova,mikalstill/nova,gooddata/openstack-nova,klmitch/nova,hanlind/nova,Juniper/nova,jianghuaw/nova,hanlind/nova,sebrandon1/nova,klmitch/nova,gooddata/openstack-nova,rajalokan/nova,rajalokan/nova,hanlind/nova,vmturbo/nova,cloudbase/nova,mahak/nova,klmitch/nova,cloudbase/nova,mikalstill/nova,gooddata/openstack-nova,rajalokan/nova,openstack/nova,rahulunair/nova,mahak/nova,jianghuaw/nova,openstack/nova,openstack/nova,mahak/nova