cmd/trace: don't fail when no browser is available
cmd/trace: don't fail when no browser is available

When there is no browser available on the system,
we should print the URL instead of failing.

Change-Id: I4a2b099e17609394273eff150062c285d76bbac1
Reviewed-on: https://go-review.googlesource.com/13774
Reviewed-by: Dmitry Vyukov <05e8181be3150e47da8fe2e1f18d381c75496ff4@google.com>

honsiorovskyi/go,CiscoCloud/go,codestation/go,constantine001/go,dterei/go,ObjectiveJoe/go,zcwfeng/go,ericsnowcurrently/go,malvira/go,chinanjjohn2012/go,deft-code/go,deft-code/go,ericsnowcurrently/go,kakuhiroshi/go,linux-on-ibm-z/go,beealone/go,kim-racktop/go,CAFxX/go,yasarkunduz/go,rentongzhang/go,xiuzhifu/go,goodlang/good,josharian/go,CiscoCloud/go,derekmarcotte/go,CAFxX/go,polyverse-security/go,roth1002/go,arschles/go-1,jdhenke/go,yangzhongj/go,famorted/go,Pulgafree/go,sndnvaps/go,ericsnowcurrently/go,rentongzhang/go,dafyddcrosby/go,qskycolor/go,chinanjjohn2012/go,theass/go,theass/go,cyberroadie/go,donnel6809/go,fjballest/golang,ichu501/go,slavau/go,Badredapple/go,Pulgafree/go,odeke-em/go,Pulgafree/go,rentongzhang/go,cloudflare/go,dterei/go,alex-zhang/go,danycoro/go,zouchao2010/go,theif519/go,kakuhiroshi/go,ichu501/go,alex-zhang/go,AALEKH/go,chinanjjohn2012/go,Jordanzuo/go,xiuzhifu/go,ifagnosticism/go,akutz/go,CarterTsai/go,lihuanghai/go,chinanjjohn2012/go,goodlang/good,yangzhongj/go,jacobhaven/go,alex-zhang/go,cyberroadie/go,gdey/go,ganboing/go-esx,jacobxk/go,zouchao2010/go,ObjectiveJoe/go,Jordanzuo/go,samanalysis/go,yasarkunduz/go,stormltf/go,Endika/go,ichu501/go,constantine001/go,roth1002/go,ganboing/go-esx,alash3al/go,Cofyc/go,Pulgafree/go,jacobxk/go,binarycrusader/go,CAFxX/go,vsekhar/elastic-go,ganboing/go-esx,CAFxX/go,undergrowthlinear/go,linux-on-ibm-z/go,uileyar/go,binarycrusader/go,miolini/go,likesea/go,theif519/go,mgyenik/go,arschles/go-1,nawawi/go,xujianhai/go,mgyenik/go,Cofyc/go,codestation/go,cookerzhu/go,suncycheng/go,dterei/go,danycoro/go,codestation/go,b54898533/go,golang/go,yangzhongj/go,ImJasonH/go,Samurais/go,zhangg/go,samanalysis/go,samanalysis/go,cyberroadie/go,Samurais/go,roth1002/go,xiuzhifu/go,b54898533/go,myPublicGit/go,slavau/go,devpadawan/go,arschles/go-1,Jordanzuo/go,ObjectiveJoe/go,zcwfeng/go,binarycrusader/go,theif519/go,monetate/go,alash3al/go,zhangg/go,yasarkunduz/go,momchil-velikov/go,theif519/go,JioCloud/go,kakuhiroshi/go,nawawi/go,leeric92/go,dafyddcrosby/go,vsekhar/elastic-go,zhangg/go,danielsoro/go,likesea/go,gdey/go,mdp/go,albertjin/golang-go,Badredapple/go,yasarkunduz/go,yangzhongj/go,zouchao2010/go,AALEKH/go,zouchao2010/go,Samurais/go,suncycheng/go,arschles/golang-go,akutz/go,xiuzhifu/go,derekmarcotte/go,jacobhaven/go,arschles/go-1,stormltf/go,qskycolor/go,beealone/go,linux-on-ibm-z/go,kim-racktop/go,yasarkunduz/go,samanalysis/go,beealone/go,skatsuta/go,cainiaocome/go,Cofyc/go,vanloswang/go,stormltf/go,mgyenik/go,slavau/go,suncycheng/go,CarterTsai/go,codestation/go,slavau/go,polyverse-security/go,zouchao2010/go,dpatel06/go,greyhwndz/go,albertjin/golang-go,josharian/go,xiaobodu/go,honsiorovskyi/go,vanloswang/go,devpadawan/go,zouchao2010/go,golang/go,stormltf/go,stormltf/go,mgyenik/go,undergrowthlinear/go,donnel6809/go,greyhwndz/go,vishsingh/go,arschles/golang-go,nawawi/go,JioCloud/go,b54898533/go,kakuhiroshi/go,albertjin/golang-go,joshsammut/go,JioCloud/go,kakuhiroshi/go,lihuanghai/go,nawawi/go,nawawi/go,CarterTsai/go,sndnvaps/go,joshsammut/go,devpadawan/go,theass/go,zcwfeng/go,alex-zhang/go,myPublicGit/go,CAFxX/go,skatsuta/go,Pulgafree/go,qskycolor/go,joshsammut/go,donnel6809/go,ifagnosticism/go,ObjectiveJoe/go,ichu501/go,akutz/go,zhangg/go,josharian/go,CiscoCloud/go,sinuos/go,vishsingh/go,sndnvaps/go,Badredapple/go,sndnvaps/go,slavau/go,undergrowthlinear/go,kim-racktop/go,NunoEdgarGub1/go,ifagnosticism/go,deft-code/go,Young55555/go,Young55555/go,alex-zhang/go,likesea/go,stormltf/go,cyberroadie/go,jacobsa/go,qskycolor/go,leeric92/go,honsiorovskyi/go,leeric92/go,stormltf/go,greyhwndz/go,undergrowthlinear/go,b54898533/go,vsdutka/go,skatsuta/go,dafyddcrosby/go,jonasi/go,leeric92/go,mwhudson/go,cainiaocome/go,zhangg/go,cainiaocome/go,miolini/go,Badredapple/go,dpatel06/go,binarycrusader/go,myPublicGit/go,GeorgiCodes/go,binarycrusader/go,lihuanghai/go,ericsnowcurrently/go,vishsingh/go,sinuos/go,Samurais/go,gdey/go,mdp/go,snowsnail/go,kim-racktop/go,Pulgafree/go,momchil-velikov/go,vishsingh/go,cloudflare/go,Cofyc/go,jonasi/go,gdey/go,rachtsingh/gogojuice,polyverse-security/go,ichu501/go,rachtsingh/gogojuice,xujianhai/go,arschles/go-1,jacobhaven/go,roth1002/go,derekmarcotte/go,slavau/go,devpadawan/go,AALEKH/go,xujianhai/go,Cofyc/go,vanloswang/go,Endika/go,sndnvaps/go,dterei/go,danycoro/go,linux-on-ibm-z/go,joshsammut/go,CiscoCloud/go,Endika/go,myPublicGit/go,myPublicGit/go,malvira/go,ifagnosticism/go,honsiorovskyi/go,likesea/go,constantine001/go,vanloswang/go,WIZARD-CXY/go,mwhudson/go,uileyar/go,odeke-em/go,CiscoCloud/go,vishsingh/go,donnel6809/go,mwhudson/go,akutz/go,theass/go,honsiorovskyi/go,skatsuta/go,mdp/go,rachtsingh/gogojuice,4ad/go,jonahglover/go,Jordanzuo/go,4ad/go,Young55555/go,mikekap/go,skatsuta/go,Samurais/go,jdhenke/go,fjballest/golang,vsekhar/elastic-go,theif519/go,polyverse-security/go,zcwfeng/go,theass/go,yangzhongj/go,dterei/go,jdhenke/go,danielsoro/go,cookerzhu/go,malvira/go,dpatel06/go,4ad/go,kakuhiroshi/go,odeke-em/go,rachtsingh/gogojuice,dpatel06/go,gdey/go,malvira/go,undergrowthlinear/go,uileyar/go,jdhenke/go,jacobxk/go,codestation/go,CAFxX/go,yangzhongj/go,Cofyc/go,deft-code/go,constantine001/go,honsiorovskyi/go,joshsammut/go,greyhwndz/go,polyverse-security/go,ObjectiveJoe/go,4ad/go,derekmarcotte/go,samanalysis/go,famorted/go,arschles/go-1,dafyddcrosby/go,ganboing/go-esx,kim-racktop/go,alash3al/go,zcwfeng/go,mgyenik/go,xujianhai/go,GeorgiCodes/go,jacobhaven/go,snowsnail/go,danielsoro/go,NunoEdgarGub1/go,danycoro/go,dpatel06/go,danielsoro/go,suncycheng/go,snowsnail/go,polyverse-security/go,ericsnowcurrently/go,mdp/go,ichu501/go,momchil-velikov/go,AALEKH/go,goodlang/good,yasarkunduz/go,monetate/go,mikekap/go,jonahglover/go,jacobsa/go,mwhudson/go,CarterTsai/go,constantine001/go,momchil-velikov/go,mdp/go,monetate/go,linux-on-ibm-z/go,Badredapple/go,jefby/go,golang/go,suncycheng/go,xujianhai/go,arschles/golang-go,derekmarcotte/go,jdhenke/go,ObjectiveJoe/go,beealone/go,GeorgiCodes/go,cyberroadie/go,ericsnowcurrently/go,NunoEdgarGub1/go,jdhenke/go,jonahglover/go,monetate/go,uileyar/go,xiaobodu/go,4ad/go,monetate/go,constantine001/go,dpatel06/go,miolini/go,rachtsingh/gogojuice,ImJasonH/go,cloudflare/go,vanloswang/go,nawawi/go,AALEKH/go,momchil-velikov/go,uileyar/go,rachtsingh/gogojuice,roth1002/go,sinuos/go,alex-zhang/go,kim-racktop/go,cloudflare/go,jefby/go,alash3al/go,cyberroadie/go,WIZARD-CXY/go,josharian/go,binarycrusader/go,xujianhai/go,CAFxX/go,likesea/go,suncycheng/go,likesea/go,Young55555/go,Young55555/go,leeric92/go,jdhenke/go,vanloswang/go,vsdutka/go,CarterTsai/go,xiuzhifu/go,suncycheng/go,deft-code/go,albertjin/golang-go,roth1002/go,ObjectiveJoe/go,jonasi/go,dterei/go,cainiaocome/go,malvira/go,lihuanghai/go,akutz/go,miolini/go,mdp/go,likesea/go,dafyddcrosby/go,b54898533/go,xiaobodu/go,albertjin/golang-go,jonasi/go,joshsammut/go,nawawi/go,samanalysis/go,jefby/go,joshsammut/go,xiuzhifu/go,jacobxk/go,xiuzhifu/go,vishsingh/go,cainiaocome/go,ganboing/go-esx,vsdutka/go,malvira/go,fjballest/golang,odeke-em/go,dterei/go,yangzhongj/go,Samurais/go,xiaobodu/go,constantine001/go,CarterTsai/go,WIZARD-CXY/go,mikekap/go,linux-on-ibm-z/go,vanloswang/go,theif519/go,chinanjjohn2012/go,lihuanghai/go,WIZARD-CXY/go,CiscoCloud/go,alash3al/go,ericsnowcurrently/go,jonasi/go,JioCloud/go,qskycolor/go,jacobsa/go,codestation/go,JioCloud/go,famorted/go,goodlang/good,devpadawan/go,albertjin/golang-go,mwhudson/go,josharian/go,goodlang/good,JioCloud/go,kakuhiroshi/go,sinuos/go,rentongzhang/go,stormltf/go,ericsnowcurrently/go,Young55555/go,Endika/go,odeke-em/go,CarterTsai/go,Jordanzuo/go,lihuanghai/go,danielsoro/go,myPublicGit/go,Endika/go,Endika/go,vanloswang/go,linux-on-ibm-z/go,vsekhar/elastic-go,jonasi/go,b54898533/go,xiaobodu/go,NunoEdgarGub1/go,ifagnosticism/go,goodlang/good,snowsnail/go,gdey/go,jonahglover/go,famorted/go,mikekap/go,GeorgiCodes/go,theass/go,ImJasonH/go,lihuanghai/go,jefby/go,uileyar/go,sinuos/go,jacobxk/go,rentongzhang/go,undergrowthlinear/go,GeorgiCodes/go,zcwfeng/go,fjballest/golang,roth1002/go,ganboing/go-esx,momchil-velikov/go,malvira/go,famorted/go,sndnvaps/go,zcwfeng/go,cyberroadie/go,vsekhar/elastic-go,josharian/go,theass/go,joshsammut/go,ifagnosticism/go,arschles/golang-go,suncycheng/go,jefby/go,akutz/go,greyhwndz/go,xujianhai/go,danielsoro/go,devpadawan/go,vishsingh/go,cainiaocome/go,ImJasonH/go,Pulgafree/go,vsekhar/elastic-go,zhangg/go,ImJasonH/go,josharian/go,momchil-velikov/go,jacobsa/go,cyberroadie/go,zhangg/go,xujianhai/go,mwhudson/go,vsdutka/go,malvira/go,mikekap/go,beealone/go,alex-zhang/go,zhangg/go,beealone/go,qskycolor/go,binarycrusader/go,snowsnail/go,dafyddcrosby/go,akutz/go,CiscoCloud/go,ganboing/go-esx,danielsoro/go,vsdutka/go,WIZARD-CXY/go,odeke-em/go,kakuhiroshi/go,mikekap/go,alash3al/go,alash3al/go,cainiaocome/go,monetate/go,deft-code/go,xiaobodu/go,Badredapple/go,codestation/go,CAFxX/go,rentongzhang/go,uileyar/go,theif519/go,jacobhaven/go,Jordanzuo/go,dpatel06/go,miolini/go,derekmarcotte/go,ImJasonH/go,monetate/go,greyhwndz/go,jacobsa/go,Young55555/go,mwhudson/go,Cofyc/go,miolini/go,Jordanzuo/go,gdey/go,slavau/go,famorted/go,arschles/golang-go,arschles/go-1,rachtsingh/gogojuice,Cofyc/go,Badredapple/go,ganboing/go-esx,4ad/go,WIZARD-CXY/go,codestation/go,mgyenik/go,jdhenke/go,Samurais/go,danycoro/go,fjballest/golang,greyhwndz/go,4ad/go,JioCloud/go,mdp/go,jefby/go,samanalysis/go,jonahglover/go,beealone/go,linux-on-ibm-z/go,vsekhar/elastic-go,Young55555/go,Pulgafree/go,derekmarcotte/go,danycoro/go,donnel6809/go,danycoro/go,goodlang/good,arschles/golang-go,honsiorovskyi/go,chinanjjohn2012/go,NunoEdgarGub1/go,cookerzhu/go,deft-code/go,Endika/go,roth1002/go,danycoro/go,beealone/go,famorted/go,ifagnosticism/go,polyverse-security/go,NunoEdgarGub1/go,cloudflare/go,jonahglover/go,NunoEdgarGub1/go,devpadawan/go,jacobhaven/go,arschles/golang-go,honsiorovskyi/go,jonasi/go,mikekap/go,vsdutka/go,xiaobodu/go,cookerzhu/go,donnel6809/go,undergrowthlinear/go,ObjectiveJoe/go,cloudflare/go,leeric92/go,CarterTsai/go,dterei/go,devpadawan/go,theass/go,WIZARD-CXY/go,snowsnail/go,sinuos/go,rentongzhang/go,mgyenik/go,CiscoCloud/go,jonahglover/go,qskycolor/go,danielsoro/go,mdp/go,monetate/go,cloudflare/go,jefby/go,GeorgiCodes/go,vsekhar/elastic-go,jacobsa/go,leeric92/go,ImJasonH/go,cloudflare/go,donnel6809/go,AALEKH/go,skatsuta/go,myPublicGit/go,binarycrusader/go,leeric92/go,mgyenik/go,ichu501/go,Samurais/go,zouchao2010/go,jonahglover/go,jacobxk/go,chinanjjohn2012/go,undergrowthlinear/go,famorted/go,Jordanzuo/go,rachtsingh/gogojuice,akutz/go,slavau/go,fjballest/golang,albertjin/golang-go,jacobxk/go,alash3al/go,b54898533/go,alex-zhang/go,miolini/go,Endika/go,momchil-velikov/go,miolini/go,skatsuta/go,4ad/go,AALEKH/go,odeke-em/go,ifagnosticism/go,golang/go,dafyddcrosby/go,WIZARD-CXY/go,gdey/go,derekmarcotte/go,likesea/go,cookerzhu/go,golang/go,christopher-henderson/Go,albertjin/golang-go,dafyddcrosby/go,chinanjjohn2012/go,sinuos/go,arschles/go-1,nawawi/go,deft-code/go,skatsuta/go,mwhudson/go,jacobsa/go,xiaobodu/go,kim-racktop/go,jacobhaven/go,vsdutka/go,kim-racktop/go,dpatel06/go,snowsnail/go,golang/go,rentongzhang/go,cainiaocome/go,constantine001/go,josharian/go,xiuzhifu/go,lihuanghai/go,sinuos/go,jonasi/go,GeorgiCodes/go,ichu501/go,b54898533/go,polyverse-security/go,yasarkunduz/go,arschles/golang-go,myPublicGit/go,cookerzhu/go,vishsingh/go,jacobxk/go,jacobsa/go,greyhwndz/go,uileyar/go,golang/go,theif519/go,mikekap/go,donnel6809/go,goodlang/good,jefby/go,zouchao2010/go,ImJasonH/go,Badredapple/go,golang/go,yasarkunduz/go,jacobhaven/go,zcwfeng/go,samanalysis/go,sndnvaps/go,qskycolor/go,GeorgiCodes/go,NunoEdgarGub1/go,fjballest/golang,JioCloud/go,vsdutka/go,cookerzhu/go,sndnvaps/go,yangzhongj/go,AALEKH/go,snowsnail/go,fjballest/golang,cookerzhu/go,odeke-em/go