jsoo: Add an Exit exception in FStar.Main
jsoo: Add an Exit exception in FStar.Main

This exception is raised by the Javascript version of Sys.exit (used when
compiling with js_of_ocaml)

FStarLang/FStar,A-Manning/FStar,A-Manning/FStar,FStarLang/FStar,A-Manning/FStar,FStarLang/FStar