Improve CopyStep's error message.
Improve CopyStep's error message.

Summary:
I saw this while developing, and I was not sure what was attempted
to be copied.

Test Plan: Sandcastle builds.

stuhood/buck,k21/buck,bocon13/buck,JoelMarcey/buck,ilya-klyuchnikov/buck,lukw00/buck,dsyang/buck,MarkRunWu/buck,romanoid/buck,vschs007/buck,Learn-Android-app/buck,dsyang/buck,sdwilsh/buck,clonetwin26/buck,mnuessler/buck,justinmuller/buck,zpao/buck,neonichu/buck,OkBuilds/buck,raviagarwal7/buck,mnuessler/buck,thinkernel/buck,Distrotech/buck,nguyentruongtho/buck,dsyang/buck,hgl888/buck,tgummerer/buck,LegNeato/buck,Distrotech/buck,mnuessler/buck,hgl888/buck,raviagarwal7/buck,raviagarwal7/buck,Distrotech/buck,shybovycha/buck,daedric/buck,Dominator008/buck,mnuessler/buck,rowillia/buck,luiseduardohdbackup/buck,lukw00/buck,ilya-klyuchnikov/buck,facebook/buck,Dominator008/buck,dsyang/buck,sdwilsh/buck,brettwooldridge/buck,brettwooldridge/buck,rowillia/buck,vine/buck,neonichu/buck,Dominator008/buck,mikekap/buck,vine/buck,kageiit/buck,rowillia/buck,vschs007/buck,SeleniumHQ/buck,luiseduardohdbackup/buck,raviagarwal7/buck,davido/buck,shs96c/buck,clonetwin26/buck,darkforestzero/buck,1yvT0s/buck,rhencke/buck,stuhood/buck,dsyang/buck,davido/buck,vschs007/buck,mogers/buck,Heart2009/buck,illicitonion/buck,stuhood/buck,thinkernel/buck,robbertvanginkel/buck,darkforestzero/buck,daedric/buck,luiseduardohdbackup/buck,mikekap/buck,mikekap/buck,Addepar/buck,luiseduardohdbackup/buck,brettwooldridge/buck,k21/buck,grumpyjames/buck,darkforestzero/buck,grumpyjames/buck,lukw00/buck,luiseduardohdbackup/buck,LegNeato/buck,mikekap/buck,shybovycha/buck,sdwilsh/buck,daedric/buck,siddhartharay007/buck,rmaz/buck,zhuxiaohao/buck,Addepar/buck,stuhood/buck,Distrotech/buck,marcinkwiatkowski/buck,dushmis/buck,Addepar/buck,liuyang-li/buck,illicitonion/buck,romanoid/buck,shs96c/buck,Distrotech/buck,clonetwin26/buck,jackminicloud/buck,SeleniumHQ/buck,mikekap/buck,Heart2009/buck,Addepar/buck,rhencke/buck,LegNeato/buck,MarkRunWu/buck,lukw00/buck,mikekap/buck,robbertvanginkel/buck,marcinkwiatkowski/buck,1yvT0s/buck,Learn-Android-app/buck,brettwooldridge/buck,Heart2009/buck,facebook/buck,darkforestzero/buck,JoelMarcey/buck,zpao/buck,Dominator008/buck,OkBuilds/buck,janicduplessis/buck,marcinkwiatkowski/buck,zpao/buck,marcinkwiatkowski/buck,vschs007/buck,justinmuller/buck,lukw00/buck,Heart2009/buck,siddhartharay007/buck,darkforestzero/buck,robbertvanginkel/buck,vine/buck,Distrotech/buck,kageiit/buck,dushmis/buck,liuyang-li/buck,Learn-Android-app/buck,artiya4u/buck,grumpyjames/buck,daedric/buck,brettwooldridge/buck,k21/buck,pwz3n0/buck,vine/buck,mnuessler/buck,justinmuller/buck,siddhartharay007/buck,stuhood/buck,1yvT0s/buck,dsyang/buck,MarkRunWu/buck,Heart2009/buck,marcinkwiatkowski/buck,belomx/open_tools,OkBuilds/buck,tgummerer/buck,illicitonion/buck,daedric/buck,darkforestzero/buck,raviagarwal7/buck,tgummerer/buck,neonichu/buck,JoelMarcey/buck,SeleniumHQ/buck,JoelMarcey/buck,dpursehouse/buck,Addepar/buck,Distrotech/buck,MarkRunWu/buck,sdwilsh/buck,davido/buck,OkBuilds/buck,lukw00/buck,pwz3n0/buck,pwz3n0/buck,robbertvanginkel/buck,romanoid/buck,nguyentruongtho/buck,mogers/buck,lukw00/buck,marcinkwiatkowski/buck,dushmis/buck,belomx/open_tools,brettwooldridge/buck,illicitonion/buck,Distrotech/buck,SeleniumHQ/buck,kageiit/buck,dpursehouse/buck,shs96c/buck,zhuxiaohao/buck,grumpyjames/buck,siddhartharay007/buck,Learn-Android-app/buck,romanoid/buck,shybovycha/buck,darkforestzero/buck,darkforestzero/buck,mread/buck,Distrotech/buck,liuyang-li/buck,dpursehouse/buck,davido/buck,zhuxiaohao/buck,bocon13/buck,clonetwin26/buck,hgl888/buck,k21/buck,zpao/buck,illicitonion/buck,rhencke/buck,mogers/buck,nguyentruongtho/buck,nguyentruongtho/buck,bocon13/buck,ilya-klyuchnikov/buck,zhuxiaohao/buck,OkBuilds/buck,janicduplessis/buck,zhuxiaohao/buck,rmaz/buck,ilya-klyuchnikov/buck,vine/buck,tgummerer/buck,raviagarwal7/buck,artiya4u/buck,siddhartharay007/buck,ilya-klyuchnikov/buck,shybovycha/buck,SeleniumHQ/buck,bocon13/buck,tgummerer/buck,janicduplessis/buck,justinmuller/buck,SeleniumHQ/buck,janicduplessis/buck,shybovycha/buck,romanoid/buck,robbertvanginkel/buck,vine/buck,sdwilsh/buck,neonichu/buck,LegNeato/buck,neonichu/buck,vschs007/buck,clonetwin26/buck,JoelMarcey/buck,luiseduardohdbackup/buck,zhuxiaohao/buck,LegNeato/buck,davido/buck,Addepar/buck,romanoid/buck,davido/buck,bocon13/buck,bocon13/buck,k21/buck,daedric/buck,robbertvanginkel/buck,vschs007/buck,janicduplessis/buck,Dominator008/buck,tgummerer/buck,sdwilsh/buck,sdwilsh/buck,shybovycha/buck,grumpyjames/buck,romanoid/buck,mogers/buck,hgl888/buck,bocon13/buck,Addepar/buck,facebook/buck,nguyentruongtho/buck,mogers/buck,SeleniumHQ/buck,vine/buck,GerritCodeReview/buck,k21/buck,clonetwin26/buck,dpursehouse/buck,neonichu/buck,pwz3n0/buck,mogers/buck,Learn-Android-app/buck,rmaz/buck,stuhood/buck,mikekap/buck,kageiit/buck,JoelMarcey/buck,artiya4u/buck,k21/buck,JoelMarcey/buck,justinmuller/buck,daedric/buck,janicduplessis/buck,sdwilsh/buck,rmaz/buck,k21/buck,Addepar/buck,robbertvanginkel/buck,1yvT0s/buck,zhuxiaohao/buck,facebook/buck,ilya-klyuchnikov/buck,kageiit/buck,nguyentruongtho/buck,clonetwin26/buck,1yvT0s/buck,bocon13/buck,hgl888/buck,Addepar/buck,vine/buck,dsyang/buck,vschs007/buck,MarkRunWu/buck,shs96c/buck,zhan-xiong/buck,janicduplessis/buck,darkforestzero/buck,stuhood/buck,clonetwin26/buck,raviagarwal7/buck,rowillia/buck,mread/buck,romanoid/buck,ilya-klyuchnikov/buck,rhencke/buck,dsyang/buck,rmaz/buck,dpursehouse/buck,Dominator008/buck,stuhood/buck,bocon13/buck,SeleniumHQ/buck,Heart2009/buck,pwz3n0/buck,grumpyjames/buck,jackminicloud/buck,illicitonion/buck,daedric/buck,rhencke/buck,sdwilsh/buck,daedric/buck,illicitonion/buck,liuyang-li/buck,siddhartharay007/buck,shybovycha/buck,Heart2009/buck,marcinkwiatkowski/buck,pwz3n0/buck,shs96c/buck,Dominator008/buck,robbertvanginkel/buck,darkforestzero/buck,clonetwin26/buck,neonichu/buck,liuyang-li/buck,1yvT0s/buck,LegNeato/buck,davido/buck,Learn-Android-app/buck,liuyang-li/buck,brettwooldridge/buck,dushmis/buck,mread/buck,davido/buck,facebook/buck,siddhartharay007/buck,grumpyjames/buck,zhan-xiong/buck,OkBuilds/buck,saleeh93/buck-cutom,Addepar/buck,k21/buck,zhan-xiong/buck,grumpyjames/buck,liuyang-li/buck,marcinkwiatkowski/buck,vschs007/buck,brettwooldridge/buck,mikekap/buck,artiya4u/buck,saleeh93/buck-cutom,Learn-Android-app/buck,facebook/buck,JoelMarcey/buck,liuyang-li/buck,luiseduardohdbackup/buck,rhencke/buck,mogers/buck,zhan-xiong/buck,tgummerer/buck,Dominator008/buck,raviagarwal7/buck,davido/buck,rmaz/buck,artiya4u/buck,rowillia/buck,shybovycha/buck,shs96c/buck,stuhood/buck,mnuessler/buck,vschs007/buck,JoelMarcey/buck,zhuxiaohao/buck,pwz3n0/buck,shybovycha/buck,raviagarwal7/buck,illicitonion/buck,clonetwin26/buck,janicduplessis/buck,daedric/buck,kageiit/buck,thinkernel/buck,shs96c/buck,vine/buck,rowillia/buck,belomx/open_tools,ilya-klyuchnikov/buck,mikekap/buck,SeleniumHQ/buck,pwz3n0/buck,clonetwin26/buck,liuyang-li/buck,rmaz/buck,daedric/buck,rowillia/buck,daedric/buck,romanoid/buck,mikekap/buck,mogers/buck,OkBuilds/buck,rhencke/buck,rowillia/buck,JoelMarcey/buck,luiseduardohdbackup/buck,vschs007/buck,pwz3n0/buck,GerritCodeReview/buck,rmaz/buck,sdwilsh/buck,hgl888/buck,LegNeato/buck,romanoid/buck,dsyang/buck,zhan-xiong/buck,clonetwin26/buck,mikekap/buck,facebook/buck,zhan-xiong/buck,dushmis/buck,liuyang-li/buck,artiya4u/buck,dsyang/buck,hgl888/buck,darkforestzero/buck,zpao/buck,mread/buck,luiseduardohdbackup/buck,nguyentruongtho/buck,dpursehouse/buck,sdwilsh/buck,rowillia/buck,stuhood/buck,shs96c/buck,davido/buck,robbertvanginkel/buck,raviagarwal7/buck,LegNeato/buck,siddhartharay007/buck,kageiit/buck,LegNeato/buck,janicduplessis/buck,Heart2009/buck,neonichu/buck,Dominator008/buck,brettwooldridge/buck,vschs007/buck,grumpyjames/buck,Distrotech/buck,Dominator008/buck,JoelMarcey/buck,JoelMarcey/buck,clonetwin26/buck,rowillia/buck,dushmis/buck,dushmis/buck,belomx/open_tools,lukw00/buck,brettwooldridge/buck,shs96c/buck,brettwooldridge/buck,lukw00/buck,1yvT0s/buck,bocon13/buck,davido/buck,shs96c/buck,vschs007/buck,mnuessler/buck,ilya-klyuchnikov/buck,davido/buck,shybovycha/buck,justinmuller/buck,1yvT0s/buck,rmaz/buck,zhan-xiong/buck,stuhood/buck,illicitonion/buck,hgl888/buck,zhan-xiong/buck,justinmuller/buck,artiya4u/buck,ilya-klyuchnikov/buck,k21/buck,shybovycha/buck,Dominator008/buck,k21/buck,OkBuilds/buck,artiya4u/buck,janicduplessis/buck,zhan-xiong/buck,artiya4u/buck,dpursehouse/buck,Addepar/buck,LegNeato/buck,justinmuller/buck,saleeh93/buck-cutom,sdwilsh/buck,LegNeato/buck,tgummerer/buck,justinmuller/buck,rmaz/buck,justinmuller/buck,grumpyjames/buck,zhan-xiong/buck,shybovycha/buck,OkBuilds/buck,Dominator008/buck,GerritCodeReview/buck,mnuessler/buck,k21/buck,mread/buck,dsyang/buck,marcinkwiatkowski/buck,dushmis/buck,saleeh93/buck-cutom,bocon13/buck,mread/buck,Learn-Android-app/buck,Learn-Android-app/buck,zpao/buck,MarkRunWu/buck,lukw00/buck,justinmuller/buck,dushmis/buck,rhencke/buck,dushmis/buck,mogers/buck,shybovycha/buck,OkBuilds/buck,vine/buck,ilya-klyuchnikov/buck,OkBuilds/buck,shs96c/buck,JoelMarcey/buck,rhencke/buck,davido/buck,raviagarwal7/buck,saleeh93/buck-cutom,marcinkwiatkowski/buck,neonichu/buck,brettwooldridge/buck,robbertvanginkel/buck,k21/buck,robbertvanginkel/buck,lukw00/buck,LegNeato/buck,SeleniumHQ/buck,sdwilsh/buck,dsyang/buck,robbertvanginkel/buck,SeleniumHQ/buck,bocon13/buck,mnuessler/buck,darkforestzero/buck,siddhartharay007/buck,tgummerer/buck,romanoid/buck,mogers/buck,rmaz/buck,mikekap/buck,marcinkwiatkowski/buck,illicitonion/buck,Heart2009/buck,romanoid/buck,Distrotech/buck,jackminicloud/buck,rhencke/buck,zhan-xiong/buck,janicduplessis/buck,mogers/buck,ilya-klyuchnikov/buck,ilya-klyuchnikov/buck,zhuxiaohao/buck,marcinkwiatkowski/buck,shs96c/buck,rmaz/buck,SeleniumHQ/buck,vine/buck,1yvT0s/buck,daedric/buck,illicitonion/buck,MarkRunWu/buck,Learn-Android-app/buck,robbertvanginkel/buck,justinmuller/buck,LegNeato/buck,justinmuller/buck,zhan-xiong/buck,marcinkwiatkowski/buck,dpursehouse/buck,rmaz/buck,darkforestzero/buck,GerritCodeReview/buck,zhan-xiong/buck,artiya4u/buck,grumpyjames/buck,liuyang-li/buck,rowillia/buck,zpao/buck,shs96c/buck,rowillia/buck,SeleniumHQ/buck,romanoid/buck,hgl888/buck,vschs007/buck,raviagarwal7/buck,Addepar/buck,thinkernel/buck,Learn-Android-app/buck,artiya4u/buck,Addepar/buck,pwz3n0/buck,saleeh93/buck-cutom,illicitonion/buck,pwz3n0/buck,raviagarwal7/buck,brettwooldridge/buck,OkBuilds/buck,janicduplessis/buck,illicitonion/buck,tgummerer/buck,MarkRunWu/buck,dsyang/buck,OkBuilds/buck,rhencke/buck,tgummerer/buck,jackminicloud/buck,grumpyjames/buck