Added a better error message in case dub was run outside of a package directory, issue #11.
Added a better error message in case dub was run outside of a package directory, issue #11.

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