Add a error check in postHijacked to avoid docker exec command blocking.
Add a error check in postHijacked to avoid docker exec command blocking.

When user execute docker exec command, docker daemon maybe have err return because
of ExecExists check, and then the hijack stream will not be close, it can lead to
docker exec command block.

Signed-off-by: yangshukui <f5d4db63b050213ca15a672a961ddf5176863aca@huawei.com>

coreos/docker,docker/docker,trebonian/docker,crosbymichael/docker,jwhonce/docker,nalind/docker,paralin/docker,moby/moby,dims/docker,yongtang/docker,cyphar/docker,jwhonce/docker,Ramzec/docker,noironetworks/docker,mahak/docker,srust/docker,jlhawn/docker,jimjag/docker,mkumatag/docker,jlhawn/docker,ibuildthecloud/docker,amatsus/docker,resin-io/docker,lixiaobing10051267/docker,whuwxl/docker,jimjag/docker,moul/docker,jlhawn/docker,whuwxl/docker,POLJM242/docker,yasiramin10p/moby,crosbymichael/docker,djs55/docker,dperny/docker,mahak/docker,dmcgowan/docker,gnailuy/docker,shishir-a412ed/docker,flavio/docker,mixja/docker,laijs/docker,dmcgowan/docker,maxamillion/docker,johnstep/moby-moby,bearstech/moby,flavio/docker,SvenDowideit/docker,konstruktoid/docker-upstream,thaJeztah/docker,vdemeester/docker,whuwxl/docker,liaoqingwei/docker,trebonian/docker,YuPengZTE/docker,nawawi/docker,nawawi/docker,jthelin/docker,unclejack/docker,coolljt0725/docker,yummypeng/docker,JMesser81/docker,mahak/docker,kolyshkin/docker,kolyshkin/moby,miminar/docker,kolyshkin/docker,bonczj/moby-json-file-logger,x1022as/docker,trebonian/docker,naveed-jamil-tenpearls/moby,sterchelen/docker,moby/moby,kolyshkin/moby,scotttlin/docker,ndeloof/docker,amatsus/docker,nawawi/docker,madhanrm/docker,vdemeester/docker,JMesser81/docker,raja-sami-10p/moby,rremer/moby,allencloud/docker,tmp6154/docker,nalind/docker,justincormack/docker,ripcurld0/moby,mixja/docker,paralin/docker,sterchelen/docker,djs55/docker,pradipd/moby,Ramzec/docker,kojiromike/docker,yosifkit/docker,jwhonce/docker,SvenDowideit/docker,PatrickLang/moby,laijs/docker,jameszhan/docker,lixiaobing1/docker,madhanrm/docker,tophj-ibm/moby,Mokolea/docker,tklauser/docker,duglin/docker,POLJM242/docker,duglin/docker,jameszhan/docker,imkin/moby,mike1729/docker,trebonian/docker,rancher/docker,YuPengZTE/docker,moul/docker,shhsu/docker,kojiromike/docker,gnailuy/docker,tophj-ibm/moby,dsheets/moby,thaJeztah/docker,pramodhkp/moby,bonczj/moby-json-file-logger,konstruktoid/docker-1,vdemeester/docker,konstruktoid/docker-upstream,fcrisciani/docker,moby/moby,charleszheng44/moby,amatsus/docker,crosbymichael/docker,KarthikNayak/docker,ibuildthecloud/docker,pramodhkp/moby,ripcurld0/moby,haveatry/docker,liaoqingwei/docker,jameszhan/docker,yosifkit/docker,laijs/moby,duglin/docker,konstruktoid/docker-upstream,ripcurld0/moby,bonczj/moby-json-file-logger,djs55/docker,KarthikNayak/docker,AkihiroSuda/docker,noironetworks/docker,ibuildthecloud/docker,shhsu/docker,danyalKhaliq10p/moby,flavio/docker,haveatry/docker,jvanz/docker,unclejack/docker,AkihiroSuda/docker,rremer/moby,raja-sami-10p/moby,scotttlin/docker,tophj-ibm/docker,estesp/docker,Mokolea/docker,tophj-ibm/moby,mkumatag/docker,KarthikNayak/docker,konstruktoid/docker-1,dmcgowan/docker,tklauser/docker,dccurtis/docker,mixja/docker,tmp6154/docker,dims/docker,yosifkit/docker,resin-io/docker,resin-io/docker,hqhq/moby,flavio/docker,x1022as/docker,dsheets/moby,lixiaobing1/docker,KingEmet/docker,dims/docker,hqhq/moby,cpuguy83/docker,yummypeng/docker,coolljt0725/docker,lixiaobing10051267/docker,estesp/docker,mikebrow/docker,jvanz/docker,KingEmet/docker,charleszheng44/moby,cyphar/docker,laijs/moby,madhanrm/docker,dccurtis/docker,gnailuy/docker,mike1729/docker,dsheets/moby,nawawi/docker,pradipd/moby,maxamillion/docker,mkumatag/docker,tophj-ibm/docker,JMesser81/docker,unclejack/docker,dperny/docker,imkin/moby,tonistiigi/docker,lixiaobing10051267/docker,srust/docker,jimjag/docker,ndeloof/docker,mikebrow/docker,tklauser/docker,yasiramin10p/moby,scotttlin/docker,coreos/docker,laijs/docker,laijs/moby,srust/docker,dperny/docker,cpuguy83/docker,KingEmet/docker,mahak/docker,coreos/docker,kolyshkin/moby,YuPengZTE/docker,PatrickLang/moby,cpuguy83/docker,rremer/moby,raja-sami-10p/moby,tmp6154/docker,danyalKhaliq10p/moby,mikebrow/docker,PatrickLang/moby,Mokolea/docker,konstruktoid/docker-upstream,docker/docker,kojiromike/docker,nalind/docker,ibuildthecloud/docker,justincormack/docker,x1022as/docker,rancher/docker,tonistiigi/docker,johnstep/moby-moby,miminar/docker,liaoqingwei/docker,allencloud/docker,paralin/docker,maxamillion/docker,AkihiroSuda/docker,SvenDowideit/docker,yongtang/docker,rancher/docker,thaJeztah/docker,docker/docker,gnailuy/docker,fcrisciani/docker,ibuildthecloud/docker,danyalKhaliq10p/moby,shishir-a412ed/docker,Ramzec/docker,moul/docker,allencloud/docker,estesp/docker,AkihiroSuda/docker,nalind/docker,haveatry/docker,pradipd/moby,moby/moby,tonistiigi/docker,yasiramin10p/moby,hqhq/moby,Ramzec/docker,rhatdan/docker,yongtang/docker,charleszheng44/moby,rhatdan/docker,Mokolea/docker,yosifkit/docker,sterchelen/docker,bearstech/moby,cpuguy83/docker,tophj-ibm/docker,jthelin/docker,naveed-jamil-tenpearls/moby,yummypeng/docker,pramodhkp/moby,thaJeztah/docker,naveed-jamil-tenpearls/moby,konstruktoid/docker-1,jimjag/docker,shishir-a412ed/docker,fcrisciani/docker,noironetworks/docker,jvanz/docker,rhatdan/docker,bearstech/moby,cyphar/docker,paralin/docker,dccurtis/docker,bearstech/moby,moul/docker,johnstep/moby-moby,miminar/docker,coolljt0725/docker,justincormack/docker,ndeloof/docker,lixiaobing1/docker,imkin/moby,shhsu/docker,mike1729/docker