utils: Teach mkdir_p to fail if the existing target isn't a directory
utils: Teach mkdir_p to fail if the existing target isn't a directory

Existing symlinks to directories will still pass, because isdir()
follows symbolic links.

kennethlyn/pelican,janaurka/git-debug-presentiation,gymglish/pelican,jimperio/pelican,deved69/pelican-1,treyhunner/pelican,lazycoder-ru/pelican,rbarraud/pelican,eevee/pelican,sunzhongwei/pelican,HyperGroups/pelican,JeremyMorgan/pelican,number5/pelican,florianjacob/pelican,ehashman/pelican,0xMF/pelican,janaurka/git-debug-presentiation,abrahamvarricatt/pelican,jimperio/pelican,douglaskastle/pelican,JeremyMorgan/pelican,karlcow/pelican,number5/pelican,kernc/pelican,ehashman/pelican,liyonghelpme/myBlog,zackw/pelican,TC01/pelican,janaurka/git-debug-presentiation,Summonee/pelican,btnpushnmunky/pelican,jvehent/pelican,zackw/pelican,GiovanniMoretti/pelican,lazycoder-ru/pelican,lucasplus/pelican,liyonghelpme/myBlog,iurisilvio/pelican,btnpushnmunky/pelican,lucasplus/pelican,florianjacob/pelican,11craft/pelican,sunzhongwei/pelican,getpelican/pelican,douglaskastle/pelican,levanhien8/pelican,Scheirle/pelican,catdog2/pelican,ls2uper/pelican,Scheirle/pelican,fbs/pelican,jvehent/pelican,garbas/pelican,deanishe/pelican,garbas/pelican,Scheirle/pelican,simonjj/pelican,levanhien8/pelican,deved69/pelican-1,jvehent/pelican,talha131/pelican,liyonghelpme/myBlog,goerz/pelican,51itclub/pelican,rbarraud/pelican,Polyconseil/pelican,gymglish/pelican,gymglish/pelican,Rogdham/pelican,HyperGroups/pelican,kernc/pelican,goerz/pelican,JeremyMorgan/pelican,koobs/pelican,number5/pelican,iurisilvio/pelican,treyhunner/pelican,catdog2/pelican,alexras/pelican,iKevinY/pelican,alexras/pelican,jo-tham/pelican,UdeskDeveloper/pelican,crmackay/pelican,Summonee/pelican,deanishe/pelican,treyhunner/pelican,crmackay/pelican,sunzhongwei/pelican,ehashman/pelican,florianjacob/pelican,ionelmc/pelican,avaris/pelican,GiovanniMoretti/pelican,GiovanniMoretti/pelican,goerz/pelican,11craft/pelican,simonjj/pelican,justinmayer/pelican,ls2uper/pelican,douglaskastle/pelican,farseerfc/pelican,eevee/pelican,karlcow/pelican,avaris/pelican,btnpushnmunky/pelican,TC01/pelican,rbarraud/pelican,lazycoder-ru/pelican,ingwinlu/pelican,51itclub/pelican,ingwinlu/pelican,UdeskDeveloper/pelican,deanishe/pelican,lucasplus/pelican,TC01/pelican,HyperGroups/pelican,Summonee/pelican,abrahamvarricatt/pelican,liyonghelpme/myBlog,abrahamvarricatt/pelican,garbas/pelican,iKevinY/pelican,sunzhongwei/pelican,levanhien8/pelican,karlcow/pelican,simonjj/pelican,farseerfc/pelican,Polyconseil/pelican,koobs/pelican,11craft/pelican,joetboole/pelican,kennethlyn/pelican,jo-tham/pelican,jimperio/pelican,catdog2/pelican,talha131/pelican,joetboole/pelican,iurisilvio/pelican,UdeskDeveloper/pelican,Rogdham/pelican,ls2uper/pelican,crmackay/pelican,arty-name/pelican,joetboole/pelican,kernc/pelican,getpelican/pelican,alexras/pelican,liyonghelpme/myBlog,zackw/pelican,eevee/pelican,51itclub/pelican,deved69/pelican-1,kennethlyn/pelican,Rogdham/pelican,koobs/pelican