Make exception message more friendly
Make exception message more friendly

Exceptions that get returned via REST api
should have more user friendly message,
users shouldn't have to know backend concept.
more detail,see bug.

Change-Id: Idc20680e46c36fa3441838177ce61b6f5e1d2979
Closes-Bug: #1263204

CEG-FYP-OpenStack/scheduler,Juniper/nova,redhat-openstack/nova,maelnor/nova,blueboxgroup/nova,luogangyi/bcec-nova,tudorvio/nova,shail2810/nova,sebrandon1/nova,alexandrucoman/vbox-nova-driver,mmnelemane/nova,klmitch/nova,viggates/nova,yosshy/nova,maelnor/nova,tealover/nova,vmturbo/nova,rahulunair/nova,openstack/nova,JioCloud/nova,rajalokan/nova,vladikr/nova_drafts,dims/nova,ewindisch/nova,devendermishrajio/nova,cernops/nova,takeshineshiro/nova,klmitch/nova,shahar-stratoscale/nova,hanlind/nova,fnordahl/nova,Tehsmash/nova,raildo/nova,Tehsmash/nova,Juniper/nova,thomasem/nova,alaski/nova,tianweizhang/nova,isyippee/nova,devendermishrajio/nova,mikalstill/nova,virtualopensystems/nova,CiscoSystems/nova,yatinkumbhare/openstack-nova,CloudServer/nova,vladikr/nova_drafts,tanglei528/nova,shahar-stratoscale/nova,NeCTAR-RC/nova,alvarolopez/nova,whitepages/nova,mahak/nova,Yusuke1987/openstack_template,klmitch/nova,bigswitch/nova,BeyondTheClouds/nova,tealover/nova,badock/nova,adelina-t/nova,blueboxgroup/nova,Metaswitch/calico-nova,gooddata/openstack-nova,spring-week-topos/nova-week,takeshineshiro/nova,tangfeixiong/nova,zhimin711/nova,scripnichenko/nova,MountainWei/nova,luogangyi/bcec-nova,CiscoSystems/nova,sebrandon1/nova,adelina-t/nova,projectcalico/calico-nova,sebrandon1/nova,shail2810/nova,Francis-Liu/animated-broccoli,jianghuaw/nova,barnsnake351/nova,j-carpentier/nova,akash1808/nova,orbitfp7/nova,silenceli/nova,Francis-Liu/animated-broccoli,JianyuWang/nova,belmiromoreira/nova,iuliat/nova,bigswitch/nova,BeyondTheClouds/nova,badock/nova,gooddata/openstack-nova,double12gzh/nova,kimjaejoong/nova,Juniper/nova,scripnichenko/nova,jianghuaw/nova,alvarolopez/nova,mahak/nova,MountainWei/nova,leilihh/novaha,petrutlucian94/nova,CloudServer/nova,akash1808/nova_test_latest,cyx1231st/nova,spring-week-topos/nova-week,eharney/nova,alexandrucoman/vbox-nova-driver,felixma/nova,Yusuke1987/openstack_template,ruslanloman/nova,virtualopensystems/nova,cloudbase/nova,mgagne/nova,LoHChina/nova,LoHChina/nova,mahak/nova,cloudbase/nova-virtualbox,gooddata/openstack-nova,cloudbase/nova,cloudbase/nova,JianyuWang/nova,cyx1231st/nova,edulramirez/nova,JioCloud/nova,angdraug/nova,cernops/nova,varunarya10/nova_test_latest,edulramirez/nova,alaski/nova,rahulunair/nova,Stavitsky/nova,nikesh-mahalka/nova,eayunstack/nova,jianghuaw/nova,dims/nova,affo/nova,petrutlucian94/nova,nikesh-mahalka/nova,leilihh/novaha,yosshy/nova,viggates/nova,tianweizhang/nova,yatinkumbhare/openstack-nova,zzicewind/nova,rrader/nova-docker-plugin,devendermishrajio/nova_test_latest,joker946/nova,eonpatapon/nova,hanlind/nova,raildo/nova,TwinkleChawla/nova,rajalokan/nova,zaina/nova,mandeepdhami/nova,redhat-openstack/nova,kimjaejoong/nova,watonyweng/nova,joker946/nova,mmnelemane/nova,saleemjaveds/https-github.com-openstack-nova,CEG-FYP-OpenStack/scheduler,orbitfp7/nova,hanlind/nova,apporc/nova,CCI-MOC/nova,phenoxim/nova,vmturbo/nova,rrader/nova-docker-plugin,eonpatapon/nova,devendermishrajio/nova_test_latest,zzicewind/nova,jeffrey4l/nova,dawnpower/nova,affo/nova,silenceli/nova,eayunstack/nova,openstack/nova,klmitch/nova,rajalokan/nova,belmiromoreira/nova,NeCTAR-RC/nova,noironetworks/nova,gooddata/openstack-nova,BeyondTheClouds/nova,vmturbo/nova,Juniper/nova,phenoxim/nova,isyippee/nova,thomasem/nova,TwinkleChawla/nova,JioCloud/nova_test_latest,eharney/nova,berrange/nova,projectcalico/calico-nova,ted-gould/nova,cloudbase/nova-virtualbox,tangfeixiong/nova,barnsnake351/nova,openstack/nova,ruslanloman/nova,bgxavier/nova,iuliat/nova,whitepages/nova,apporc/nova,Metaswitch/calico-nova,tanglei528/nova,mandeepdhami/nova,bgxavier/nova,akash1808/nova,jianghuaw/nova,jeffrey4l/nova,ewindisch/nova,double12gzh/nova,leilihh/nova,berrange/nova,CCI-MOC/nova,rahulunair/nova,saleemjaveds/https-github.com-openstack-nova,mikalstill/nova,cernops/nova,vmturbo/nova,watonyweng/nova,zhimin711/nova,dawnpower/nova,felixma/nova,fnordahl/nova,leilihh/nova,ted-gould/nova,rajalokan/nova,mgagne/nova,mikalstill/nova,varunarya10/nova_test_latest,j-carpentier/nova,noironetworks/nova,angdraug/nova,tudorvio/nova,Stavitsky/nova,akash1808/nova_test_latest,JioCloud/nova_test_latest,zaina/nova