Fix SLO test with old simplejson
Fix SLO test with old simplejson

Our requirements.txt contains "simplejson>=2.0.9". With that version,
attempting to decode invalid JSON raises ValueError, not
simplejson.JSONDecodeError. In fact, there is no such thing as
JSONDecodeError; it isn't introduced until a later version. Also,
stdlib's json module doesn't have a JSONDecodeError either. However,
JSONDecodeError inherits from ValueError, so we can just catch that
instead.

Also added a better failure message when invalid JSON is detected;
we'll now fail immediately instead of later in some assertEqual().

Change-Id: I7f0db20b2d72e89c961cb43daa6888ecd878f769

gold3bear/swift,Seagate/swift,nadeemsyed/swift,hbhdytf/mac2,nadeemsyed/swift,larsbutler/swift,hurricanerix/swift,Akanoa/swift,hbhdytf/mac,Khushbu27/Tutorial,scality/ScalitySproxydSwift,bkolli/swift,aerwin3/swift,hurricanerix/swift,aerwin3/swift,clayg/swift,swiftstack/swift,AfonsoFGarcia/swift,dencaval/swift,tipabu/swift,bradleypj823/swift,larsbutler/swift,wenhuizhang/swift,Em-Pan/swift,tipabu/swift,revoer/keystone-8.0.0,maginatics/swift,shibaniahegde/OpenStak_swift,prashanthpai/swift,levythu/swift,smerritt/swift,revoer/keystone-8.0.0,matthewoliver/swift,Khushbu27/Tutorial,shibaniahegde/OpenStak_swift,tipabu/swift,rackerlabs/swift,psachin/swift,hurricanerix/swift,maginatics/swift,notmyname/swift,thiagodasilva/swift,bkolli/swift,openstack/swift,mjzmjz/swift,scality/ScalitySproxydSwift,IPVL/swift-kilo,clayg/swift,Seagate/swift,takeshineshiro/swift,williamthegrey/swift,mjwtom/swift,dencaval/swift,nadeemsyed/swift,bouncestorage/swift,redhat-openstack/swift,redhat-openstack/swift,hbhdytf/mac2,iostackproject/IO-Bandwidth-Differentiation,thiagodasilva/swift,prashanthpai/swift,gold3bear/swift,sarvesh-ranjan/swift,notmyname/swift,wenhuizhang/swift,swiftstack/swift,redbo/swift,eatbyte/Swift,matthewoliver/swift,bouncestorage/swift,rackerlabs/swift,williamthegrey/swift,daasbank/swift,daasbank/swift,mjzmjz/swift,openstack/swift,redbo/swift,iostackproject/IO-Bandwidth-Differentiation,smerritt/swift,levythu/swift,smerritt/swift,AfonsoFGarcia/swift,hbhdytf/mac2,dpgoetz/swift,openstack/swift,smerritt/swift,zackmdavis/swift,psachin/swift,dpgoetz/swift,anishnarang/gswift,swiftstack/swift,hbhdytf/mac,bradleypj823/swift,psachin/swift,matthewoliver/swift,mjwtom/swift,sarvesh-ranjan/swift,zackmdavis/swift,eatbyte/Swift,clayg/swift,notmyname/swift,anishnarang/gswift,notmyname/swift,nadeemsyed/swift,clayg/swift,hbhdytf/mac2,Em-Pan/swift,takeshineshiro/swift,hurricanerix/swift,tipabu/swift,matthewoliver/swift,IPVL/swift-kilo,psachin/swift,openstack/swift,Akanoa/swift