ML backend : Make error message for build-config parsing nicer
ML backend : Make error message for build-config parsing nicer

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