Add an explanation message for exit code -9.
Add an explanation message for exit code -9.

This is a common enough issue to warrant a little diagnostic assistance (the compiler/linker process getting KILLed by the kernel due to out-of-memory).

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