Print error when /index.html is zero-length
Print error when /index.html is zero-length

Also print warning (in verbose mode) for other empty pages.

See #947

mdhender/hugo,mitchchn/hugo,tischda/hugo,ffrizzo/hugo,MarkDBlackwell/hugo,Zenithar/hugo,gohugoio/hugo,fedelibre/hugo,Zenithar/hugo,derekperkins/hugo,tubo028/hugo,ifraixedes/hugo,bogem/hugo,ffrizzo/hugo,fedelibre/hugo,abourget/hugo,fedelibre/hugo,tubo028/hugo,soundrussian/hugo,fhergomez/hugo,tatsushid/hugo,jygastaud/hugo,mdhender/hugo,mdhender/hugo,sprig/hugo,bogem/hugo,melo/hugo,mayah/hugo,fhergomez/hugo,tischda/hugo,digitalcraftsman/hugo,coderzh/hugo,fedelibre/hugo,vbatoufflet/hugo,abourget/hugo,mitchchn/hugo,gohugoio/hugo,halostatue/hugo,soundrussian/hugo,anthonyfok/hugo,coderzh/hugo,bramp/hugo,anthonyfok/hugo,vbatoufflet/hugo,halostatue/hugo,mdhender/hugo,jygastaud/hugo,sprig/hugo,tatsushid/hugo,derekperkins/hugo,bramp/hugo,MarkDBlackwell/hugo,mayah/hugo,digitalcraftsman/hugo,ifraixedes/hugo,melo/hugo,gohugoio/hugo,anthonyfok/hugo,kujohn/hugo,kujohn/hugo