plugin/store.Get: return a specific error if plugin is disabled
plugin/store.Get: return a specific error if plugin is disabled

Previously, a 'plugin not found' error would be returned if a plugin to be
retrieved was found but disabled. This was misleading and incorrect. Now,
a new error plugin.ErrDisabled is returned in this case. This makes the
error message when trying to statically start plugins (from daemon.json or
dockerd command line) accurate.

Signed-off-by: David Sheets <aa5a275c249287abd0efa0c6aac4f0ceae1404c4@docker.com>

ripcurld0/moby,rremer/moby,sterchelen/docker,dperny/docker,dims/docker,nalind/docker,estesp/docker,Ramzec/docker,dsheets/moby,whuwxl/docker,sterchelen/docker,jthelin/docker,pramodhkp/moby,tmp6154/docker,yummypeng/docker,KingEmet/docker,gnailuy/docker,coolljt0725/docker,raja-sami-10p/moby,ibuildthecloud/docker,shishir-a412ed/docker,fcrisciani/docker,hqhq/moby,fcrisciani/docker,jlhawn/docker,madhanrm/docker,konstruktoid/docker-upstream,AkihiroSuda/docker,tophj-ibm/moby,yosifkit/docker,laijs/docker,maxamillion/docker,dmcgowan/docker,ibuildthecloud/docker,moby/moby,whuwxl/docker,lixiaobing10051267/docker,mikebrow/docker,tmp6154/docker,PatrickLang/moby,unclejack/docker,mike1729/docker,SvenDowideit/docker,imkin/moby,tklauser/docker,allencloud/docker,SvenDowideit/docker,mixja/docker,konstruktoid/docker-1,raja-sami-10p/moby,jlhawn/docker,nalind/docker,dims/docker,yongtang/docker,srust/docker,miminar/docker,dperny/docker,laijs/docker,moul/docker,amatsus/docker,miminar/docker,SvenDowideit/docker,KarthikNayak/docker,jameszhan/docker,thaJeztah/docker,POLJM242/docker,coolljt0725/docker,bearstech/moby,bonczj/moby-json-file-logger,amatsus/docker,cpuguy83/docker,cyphar/docker,moul/docker,ndeloof/docker,dccurtis/docker,trebonian/docker,jvanz/docker,YuPengZTE/docker,bearstech/moby,srust/docker,mikebrow/docker,mike1729/docker,resin-io/docker,x1022as/docker,cpuguy83/docker,srust/docker,maxamillion/docker,allencloud/docker,ibuildthecloud/docker,kojiromike/docker,jameszhan/docker,ndeloof/docker,noironetworks/docker,pradipd/moby,PatrickLang/moby,cyphar/docker,tklauser/docker,rremer/moby,vdemeester/docker,shishir-a412ed/docker,KingEmet/docker,estesp/docker,imkin/moby,pramodhkp/moby,gnailuy/docker,pradipd/moby,lixiaobing10051267/docker,shhsu/docker,tophj-ibm/docker,Ramzec/docker,lixiaobing10051267/docker,tonistiigi/docker,johnstep/moby-moby,danyalKhaliq10p/moby,justincormack/docker,duglin/docker,kolyshkin/docker,thaJeztah/docker,noironetworks/docker,tophj-ibm/docker,dims/docker,estesp/docker,hqhq/moby,duglin/docker,tklauser/docker,nalind/docker,naveed-jamil-tenpearls/moby,sterchelen/docker,kojiromike/docker,AkihiroSuda/docker,KarthikNayak/docker,bonczj/moby-json-file-logger,tmp6154/docker,naveed-jamil-tenpearls/moby,cyphar/docker,paralin/docker,dmcgowan/docker,AkihiroSuda/docker,cpuguy83/docker,x1022as/docker,gnailuy/docker,dmcgowan/docker,docker/docker,flavio/docker,bonczj/moby-json-file-logger,KingEmet/docker,fcrisciani/docker,moul/docker,lixiaobing1/docker,rhatdan/docker,scotttlin/docker,liaoqingwei/docker,konstruktoid/docker-1,flavio/docker,jimjag/docker,tophj-ibm/docker,yummypeng/docker,kojiromike/docker,flavio/docker,laijs/moby,jvanz/docker,kolyshkin/moby,POLJM242/docker,laijs/moby,rhatdan/docker,paralin/docker,docker/docker,tophj-ibm/moby,vdemeester/docker,unclejack/docker,allencloud/docker,mixja/docker,dsheets/moby,moby/moby,bearstech/moby,Mokolea/docker,johnstep/moby-moby,Mokolea/docker,scotttlin/docker,danyalKhaliq10p/moby,mixja/docker,charleszheng44/moby,nawawi/docker,rancher/docker,noironetworks/docker,mkumatag/docker,konstruktoid/docker-1,ripcurld0/moby,moby/moby,yosifkit/docker,madhanrm/docker,cpuguy83/docker,jwhonce/docker,nawawi/docker,Ramzec/docker,jimjag/docker,AkihiroSuda/docker,moby/moby,nawawi/docker,jlhawn/docker,amatsus/docker,liaoqingwei/docker,madhanrm/docker,mkumatag/docker,nawawi/docker,ibuildthecloud/docker,YuPengZTE/docker,x1022as/docker,nalind/docker,mahak/docker,ibuildthecloud/docker,hqhq/moby,vdemeester/docker,JMesser81/docker,Mokolea/docker,docker/docker,flavio/docker,jimjag/docker,djs55/docker,charleszheng44/moby,yongtang/docker,unclejack/docker,coreos/docker,jimjag/docker,rancher/docker,JMesser81/docker,Ramzec/docker,Mokolea/docker,haveatry/docker,raja-sami-10p/moby,pradipd/moby,mkumatag/docker,johnstep/moby-moby,konstruktoid/docker-upstream,dsheets/moby,yosifkit/docker,mahak/docker,shhsu/docker,yummypeng/docker,PatrickLang/moby,bearstech/moby,thaJeztah/docker,mikebrow/docker,thaJeztah/docker,tophj-ibm/moby,resin-io/docker,moul/docker,haveatry/docker,yosifkit/docker,yongtang/docker,JMesser81/docker,coolljt0725/docker,shishir-a412ed/docker,liaoqingwei/docker,rremer/moby,YuPengZTE/docker,coreos/docker,miminar/docker,justincormack/docker,KarthikNayak/docker,pramodhkp/moby,laijs/moby,charleszheng44/moby,ndeloof/docker,dperny/docker,justincormack/docker,paralin/docker,dccurtis/docker,konstruktoid/docker-upstream,danyalKhaliq10p/moby,lixiaobing1/docker,rhatdan/docker,djs55/docker,trebonian/docker,rancher/docker,jvanz/docker,kolyshkin/docker,crosbymichael/docker,shhsu/docker,lixiaobing1/docker,tonistiigi/docker,jthelin/docker,coreos/docker,jameszhan/docker,imkin/moby,duglin/docker,maxamillion/docker,kolyshkin/moby,djs55/docker,mahak/docker,jwhonce/docker,crosbymichael/docker,naveed-jamil-tenpearls/moby,tonistiigi/docker,paralin/docker,scotttlin/docker,trebonian/docker,mike1729/docker,haveatry/docker,resin-io/docker,mahak/docker,whuwxl/docker,dccurtis/docker,konstruktoid/docker-upstream,trebonian/docker,kolyshkin/moby,ripcurld0/moby,gnailuy/docker,crosbymichael/docker,jwhonce/docker,laijs/docker