Fix error message when running DUB on a folder with no package. Fixes #696.
Fix error message when running DUB on a folder with no package. Fixes #696.

jelmansouri/dub,jeanbaptistelab/dub,xentec/dub,gedaiu/dub,grogancolin/dub,rjframe/dub,p0nce/dub,Geod24/dub,D-Programming-Language/dub,Flamaros/dub