libmachine: return an error if DOCKER_HOST is empty
libmachine: return an error if DOCKER_HOST is empty

Signed-off-by: Soshi Katsuta <37be56e1b0b2d8665f687408a43d44ca133648fd@gmail.com>

londoncalling/docker.github.io,soleo/machine,alindeman/machine,joaofnfernandes/docker.github.io,Parallels/docker-machine,denmat/machine,danix800/docker.github.io,shubheksha/docker.github.io,ksasi/machine,dweomer/docker-machine,emsi/machine,LuisBosquez/docker.github.io,voutilad/machine,xinxian0458/machine,yut148/machine,sausheong/machine,askb/machine,errordeveloper/machine,robinbb/machine,blueyed/machine,alexisbellido/docker.github.io,bookerzzz/machine,NeilW/machine,vovimayhem/machine,JimGalasyn/docker.github.io,jhoos/machine,londoncalling/machine,aduermael/docker.github.io,bwahn/machine,tpires/machine,shubheksha/docker.github.io,dnephin/machine,MHBauer/machine,aduermael/docker.github.io,danix800/docker.github.io,ppadala/machine,xiaohui/machine,johnstep/docker.github.io,docker/docker.github.io,joaofnfernandes/docker.github.io,trecloux/machine,dudymas/machine,docker/docker.github.io,rillig/docker.github.io,legal90/machine,cab105/machine,binario200/machine,unodba/machine,jhoos/machine,slowteetoe/machine,faijaz/docker-machine,yanzay/machine,dudymas/machine,robinbb/machine,anweiss/docker.github.io,tyrken/machine,denverdino/denverdino.github.io,dudymas/machine,dgageot/machine,cgvarela/machine,jgsqware/machine,beni55/machine,gdevillele/docker.github.io,slashk/machine,unodba/machine,beni55/machine,dskora/machine,shin-/docker.github.io,miqui/machine,joeuo/docker.github.io,BSWANG/denverdino.github.io,troy0820/docker.github.io,gerred/machine,menglingwei/denverdino.github.io,binario200/machine,rdgreis/machine,sausheong/machine,miqui/machine,paultag/machine,rwilliams/machine,jmelis/machine,juliengarcia/machine,abstractitptyltd/machine,denverdino/machine,imikushin/machine,xiaohui/machine,londoncalling/docker.github.io,menglingwei/denverdino.github.io,denverdino/denverdino.github.io,shin-/docker.github.io,titanlien/machine,jzwlqx/denverdino.github.io,joeuo/docker.github.io,yanzay/machine,hironobu-s/machine,databus23/machine,pottava/machine,wlan0/machine,BSWANG/denverdino.github.io,xinxian0458/machine,Grynn/machine,tyrken/machine,Herbis/machine,dskora/machine,shot/machine,johnstep/docker.github.io,BSWANG/denverdino.github.io,deitch/machine,voutilad/machine,wzymaster/machine,trecloux/machine,OpenNebula/machine,jeanlaurent/machine,MHBauer/machine,NeilW/machine,deitch/machine,denverdino/docker.github.io,13W/machine,mjtrefethen/machine,troy0820/docker.github.io,aduermael/docker.github.io,denverdino/docker.github.io,joelea/machine,cgvarela/machine,denverdino/machine-1,ddaze/machine,rwilliams/machine,databus23/machine,pragkent/machine,usmanismail/machine,menglingwei/machine,Grynn/machine,otherjason/machine,zchee/machine,bgokden/machine,thaJeztah/docker.github.io,titanlien/machine,imikushin/machine,ibuildthecloud/machine,cvstebut/machine,cvstebut/machine,rawkode/machine,tyrken/machine,sanscontext/docker.github.io,robertbreker/machine,anweiss/docker.github.io,anshulr/machine,errordeveloper/machine,denverdino/docker.github.io,JohnKDay/machine,gdevillele/docker.github.io,shubheksha/docker.github.io,phiroict/docker,alexisbellido/docker.github.io,rillig/docker.github.io,menglingwei/denverdino.github.io,asalhamed/machine,skatsuta/machine,docker/machine,robvanmieghem/machine,kunalkushwaha/machine,joeuo/docker.github.io,wenlock/machine,yanzay/machine,carolynvs/machine,juliengarcia/machine,ubiquityhosting/machine,gerred/machine,JimGalasyn/docker.github.io,cab105/machine,denverdino/machine,vovimayhem/machine,tpires/machine,docker/docker.github.io,docker-zh/docker.github.io,askb/machine,shubheksha/docker.github.io,faijaz/docker-machine,slashk/machine,ubiquityhosting/machine,bgokden/machine,LuisBosquez/docker.github.io,kacole2/machine,jzwlqx/denverdino.github.io,NeilW/machine,anweiss/docker.github.io,schatt/machine,ehazlett/machine,menglingwei/denverdino.github.io,rillig/docker.github.io,robertbreker/machine,moxiegirl/machine,joaofnfernandes/docker.github.io,ddaze/machine,denverdino/denverdino.github.io,databus23/machine,cab105/machine,sebadiaz/machine,ibuildthecloud/machine,jeanlaurent/machine,shubheksha/docker.github.io,flavio/machine,sjfloat/machine,mark-adams/machine,blueyed/machine,jarosser06/machine,robvanmieghem/machine,phiroict/docker,hironobu-s/machine,bdwill/docker.github.io,hypriot/machine,carolynvs/machine,anweiss/docker.github.io,thaJeztah/docker.github.io,vovimayhem/machine,exoscale/docker-machine,joelea/machine,jgsqware/machine,juliengarcia/machine,emsi/machine,otherjason/machine,shin-/docker.github.io,ehazlett/machine,kechol/machine,kunalkushwaha/machine,JimGalasyn/docker.github.io,mark-adams/machine,thaJeztah/machine,robvanmieghem/machine,marsmensch/machine,JimGalasyn/docker.github.io,xiaohui/machine,JohnKDay/machine,ppadala/machine,anweiss/docker.github.io,pdxjohnny/machine,jpressnell/machine,13W/machine,davidstosik/machine,clintonskitson/machine,euank/machine,rowancarr/machine,Grynn/machine,wenlock/machine,Parallels/docker-machine,bdwill/docker.github.io,jkingyens/machine,StackPointCloud/machine,kechol/machine,danix800/docker.github.io,docker-zh/docker.github.io,denverdino/machine,pragkent/machine,errordeveloper/machine,alexisbellido/docker.github.io,bdwill/docker.github.io,asalhamed/machine,ksasi/machine,denverdino/machine-1,alindeman/machine,ocean/machine,paultag/machine,carolynvs/machine,aduermael/docker.github.io,skippbox/kmachine,robertbreker/machine,jsirois/machine,denmat/machine,jsirois/machine,docker-zh/docker.github.io,csakoda/machine,davidstosik/machine,danix800/docker.github.io,gerred/machine,mjtrefethen/machine,docker-zh/docker.github.io,bwahn/machine,shin-/docker.github.io,jarosser06/machine,legal90/machine,kunalkushwaha/machine,thaJeztah/machine,jeffellin/machine,BSWANG/denverdino.github.io,jhoos/machine,zchee/machine,atsaki/machine,sjfloat/machine,phiroict/docker,jzwlqx/denverdino.github.io,bdwill/docker.github.io,rwilliams/machine,cvstebut/machine,carloreggiani/machine,BSWANG/denverdino.github.io,sjfloat/machine,rdgreis/machine,ubiquityhosting/machine,justintung/machine,bwahn/machine,jkingyens/machine,jgsqware/machine,sanscontext/docker.github.io,bookerzzz/machine,thaJeztah/machine,docker-zh/docker.github.io,johnstep/docker.github.io,jeffellin/machine,OpenNebula/machine,usmanismail/machine,londoncalling/machine,LuisBosquez/docker.github.io,laveolu/machine,jpressnell/machine,rowancarr/machine,carloreggiani/machine,menglingwei/denverdino.github.io,londoncalling/docker.github.io,marsmensch/machine,j3tm0t0/machine,rowancarr/machine,bdwill/docker.github.io,whilp/machine,bgokden/machine,JimGalasyn/docker.github.io,thaJeztah/docker.github.io,shot/machine,exoscale/docker-machine,sanscontext/docker.github.io,johnstep/docker.github.io,denverdino/docker.github.io,joaofnfernandes/docker.github.io,StackPointCloud/machine,wlan0/machine,bookerzzz/machine,flavio/machine,abstractitptyltd/machine,jzwlqx/denverdino.github.io,jlusiardi/machine,sanscontext/docker.github.io,rawkode/machine,dweomer/docker-machine,brandoncc/machine,atsaki/machine,j3tm0t0/machine,gdevillele/docker.github.io,zchee/machine,LuisBosquez/docker.github.io,pdxjohnny/machine,hironobu-s/machine,londoncalling/docker.github.io,tpires/machine,titanlien/machine,trecloux/machine,dnephin/machine,laveolu/machine,wzymaster/machine,clintonskitson/machine,jsirois/machine,wenlock/machine,davidstosik/machine,surgeforward/machine,troy0820/docker.github.io,whilp/machine,londoncalling/docker.github.io,hypriot/machine,denverdino/docker.github.io,docker/docker.github.io,shin-/docker.github.io,sebadiaz/machine,thaJeztah/docker.github.io,docker/docker.github.io,jlusiardi/machine,justintung/machine,johnstep/docker.github.io,sebadiaz/machine,wlan0/machine,pdxjohnny/machine,denverdino/denverdino.github.io,joeuo/docker.github.io,kacole2/machine,rillig/docker.github.io,thaJeztah/docker.github.io,surgeforward/machine,joeuo/docker.github.io,allingeek/machine,Kast0rTr0y/docker-machine,menglingwei/machine,phiroict/docker,schatt/machine,sanscontext/docker.github.io,jlusiardi/machine,gdevillele/docker.github.io,Herbis/machine,larsla/machine,ksasi/machine,miqui/machine,troy0820/docker.github.io,allingeek/machine,kacole2/machine,SvenDowideit/machine,askb/machine,moxiegirl/machine,pragkent/machine,jeanlaurent/machine,denverdino/denverdino.github.io,allingeek/machine,beni55/machine,Kast0rTr0y/docker-machine,menglingwei/machine,usmanismail/machine,atsaki/machine,shot/machine,soleo/machine,xinxian0458/machine,soleo/machine,jkingyens/machine,yut148/machine,skatsuta/machine,jmelis/machine,pottava/machine,joaofnfernandes/docker.github.io,jmelis/machine,wzymaster/machine,JohnKDay/machine,LuisBosquez/docker.github.io,euank/machine,OpenNebula/machine,faijaz/docker-machine,brandoncc/machine,docker/machine,jzwlqx/denverdino.github.io,SvenDowideit/machine,pottava/machine,larsla/machine,gdevillele/docker.github.io,ppadala/machine,ocean/machine,slowteetoe/machine,anshulr/machine,dgageot/machine,alexisbellido/docker.github.io,otherjason/machine,csakoda/machine,skatsuta/machine,skippbox/kmachine,moxiegirl/machine,phiroict/docker,alexisbellido/docker.github.io,StackPointCloud/machine,jeffellin/machine