Stack.Docker: use ReadProcessException as necessary
Stack.Docker: use ReadProcessException as necessary

@manny-fp: Without this change, we don't get the helpful "please use
stack docker pull" message.

MichielDerhaeg/stack,Denommus/stack,soenkehahn/stack,cocreature/stack,anton-dessiatov/stack,mathhun/stack,chadbrewbaker/stack,Heather/stack,cabrera/stack,fizruk/stack,Denommus/stack,athanclark/stack,CRogers/stack,rrnewton/stack,harendra-kumar/stack,Fuuzetsu/stack,supermario/stack,ant1441/stack,deech/stack,akhileshs/stack,DanielG/stack,sinelaw/stack,phadej/stack,mrkkrp/stack,luigy/stack,agrafix/stack,agrafix/stack,rvion/stack,joozek78/stack,achernyak/stack,chreekat/stack,meiersi-11ce/stack,CRogers/stack,GaloisInc/stack,Andrew-the-Ubergeek/stack,k-bx/stack,keent/stack,narrative/stack,bixuanzju/stack,supermario/stack,Heather/stack,bollu/stack,Andrew-the-Ubergeek/stack,lukexi/stack,vigoo/stack,markus1189/stack,duplode/stack,Blaisorblade/stack,ant1441/stack,juhp/stack,AndrewRademacher/stack,wolftune/stack,AndreasPK/stack,achernyak/stack,meiersi-11ce/stack,cabrera/stack,martin-kolinek/stack,wskplho/stack,harendra-kumar/stack,adinapoli/stack,wolftune/stack,rekka/stack,rekka/stack,vigoo/stack,tedkornish/stack,cocreature/stack,AndrewRademacher/stack,joozek78/stack,hesselink/stack,lukexi/stack,chadbrewbaker/stack,sinelaw/stack,MichielDerhaeg/stack,soenkehahn/stack,robstewart57/stack,fizruk/stack,MaxGabriel/stack,Blaisorblade/stack,sjakobi/stack,robstewart57/stack,bixuanzju/stack,chreekat/stack,akhileshs/stack,GaloisInc/stack,mrkkrp/stack,narrative/stack,mathhun/stack,adinapoli/stack,duplode/stack,tedkornish/stack,rvion/stack,martin-kolinek/stack,markus1189/stack,phadej/stack,keent/stack,anton-dessiatov/stack,kairuku/stack,AndreasPK/stack,bollu/stack,juhp/stack,athanclark/stack,sjakobi/stack,kairuku/stack,rrnewton/stack,DanielG/stack,deech/stack,luigy/stack,MaxGabriel/stack,k-bx/stack,wskplho/stack,rubik/stack,rubik/stack,Fuuzetsu/stack