Make error message for build-config parsing nicer
Make error message for build-config parsing nicer

FStarLang/FStar,YaccConstructor/FStar,hoheinzollern/FStar,hoheinzollern/FStar,A-Manning/FStar,A-Manning/FStar,A-Manning/FStar,MallabaevAzamat/FStar,YaccConstructor/FStar,hoheinzollern/FStar,hoheinzollern/FStar,YaccConstructor/FStar,MallabaevAzamat/FStar,YaccConstructor/FStar,MallabaevAzamat/FStar,FStarLang/FStar,FStarLang/FStar,MallabaevAzamat/FStar