Fix bad error message for path based dependencies with no package description.
Fix bad error message for path based dependencies with no package description.

See #535.

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