Better errors for mkdirp failure in mirror.
Better errors for mkdirp failure in mirror.

EmreAtes/spack,skosukhin/spack,krafczyk/spack,LLNL/spack,matthiasdiener/spack,iulian787/spack,LLNL/spack,iulian787/spack,tmerrick1/spack,TheTimmy/spack,skosukhin/spack,LLNL/spack,lgarren/spack,skosukhin/spack,matthiasdiener/spack,skosukhin/spack,tmerrick1/spack,EmreAtes/spack,krafczyk/spack,lgarren/spack,EmreAtes/spack,EmreAtes/spack,matthiasdiener/spack,TheTimmy/spack,LLNL/spack,tmerrick1/spack,skosukhin/spack,TheTimmy/spack,matthiasdiener/spack,mfherbst/spack,mfherbst/spack,iulian787/spack,mfherbst/spack,iulian787/spack,TheTimmy/spack,LLNL/spack,tmerrick1/spack,TheTimmy/spack,lgarren/spack,krafczyk/spack,krafczyk/spack,EmreAtes/spack,lgarren/spack,tmerrick1/spack,mfherbst/spack,iulian787/spack,matthiasdiener/spack,krafczyk/spack,lgarren/spack,mfherbst/spack