Improve error message in case of missing packages.
Improve error message in case of missing packages.

nazriel/dub,schuetzm/dub,gedaiu/dub,p0nce/dub,rjframe/dub,dreamsxin/dub,Abscissa/dub-data-mod,jelmansouri/dub,y12uc231/dub,D-Programming-Language/dub,Abscissa/dub,xentec/dub,Geod24/dub,grogancolin/dub,jeanbaptistelab/dub,Flamaros/dub,alexeibs/dub