Ensure exc message is included in user_message
Ensure exc message is included in user_message

In exception.handle the user_message was being overridden so
failed to include the exception message even if %(exc)s was in
the message. This patch ensures the correct user_message is
produced by exception.handle()

Change-Id: I2bcc041a2f8a3b929c1acbbf3d3ff1e76b5646e9
Closes-Bug: #1493025

redhat-cip/horizon,FNST-OpenStack/horizon,django-leonardo/horizon,takeshineshiro/horizon,yeming233/horizon,davidcusatis/horizon,maestro-hybrid-cloud/horizon,maestro-hybrid-cloud/horizon,philoniare/horizon,yeming233/horizon,noironetworks/horizon,takeshineshiro/horizon,gerrive/horizon,NeCTAR-RC/horizon,damien-dg/horizon,Tesora/tesora-horizon,dan1/horizon-x509,bac/horizon,FNST-OpenStack/horizon,openstack/horizon,gerrive/horizon,wolverineav/horizon,philoniare/horizon,bigswitch/horizon,damien-dg/horizon,takeshineshiro/horizon,idjaw/horizon,dan1/horizon-x509,saydulk/horizon,philoniare/horizon,sandvine/horizon,Tesora/tesora-horizon,saydulk/horizon,openstack/horizon,Mirantis/mos-horizon,Mirantis/mos-horizon,philoniare/horizon,yeming233/horizon,BiznetGIO/horizon,damien-dg/horizon,maestro-hybrid-cloud/horizon,anthonydillon/horizon,coreycb/horizon,BiznetGIO/horizon,wolverineav/horizon,Dark-Hacker/horizon,sandvine/horizon,bigswitch/horizon,FNST-OpenStack/horizon,NeCTAR-RC/horizon,wolverineav/horizon,gerrive/horizon,django-leonardo/horizon,Mirantis/mos-horizon,dan1/horizon-x509,FNST-OpenStack/horizon,redhat-cip/horizon,BiznetGIO/horizon,Mirantis/mos-horizon,redhat-openstack/horizon,redhat-cip/horizon,davidcusatis/horizon,ChameleonCloud/horizon,bac/horizon,NeCTAR-RC/horizon,BiznetGIO/horizon,django-leonardo/horizon,redhat-openstack/horizon,redhat-openstack/horizon,bac/horizon,noironetworks/horizon,coreycb/horizon,ChameleonCloud/horizon,anthonydillon/horizon,openstack/horizon,bigswitch/horizon,idjaw/horizon,openstack/horizon,coreycb/horizon,noironetworks/horizon,NeCTAR-RC/horizon,Tesora/tesora-horizon,noironetworks/horizon,sandvine/horizon,redhat-openstack/horizon,bac/horizon,Tesora/tesora-horizon,takeshineshiro/horizon,gerrive/horizon,anthonydillon/horizon,dan1/horizon-x509,ChameleonCloud/horizon,damien-dg/horizon,anthonydillon/horizon,coreycb/horizon,wolverineav/horizon,redhat-cip/horizon,idjaw/horizon,yeming233/horizon,bigswitch/horizon,ChameleonCloud/horizon,Dark-Hacker/horizon,Dark-Hacker/horizon,saydulk/horizon,Dark-Hacker/horizon,idjaw/horizon,django-leonardo/horizon,saydulk/horizon,davidcusatis/horizon,davidcusatis/horizon,maestro-hybrid-cloud/horizon,sandvine/horizon