lib: add field `Target.if_fails_activate`
lib: create and propagate `State.make_target_die`
lib: activate `if_fails_activate` targets
test: add a case of `if_fails_activate` usage
lib: propagate `if_fails_activate` in the EDSL
test: use `if_fails_activate` in website deployment
lib: display and follow fallback-targets
Merge branch `if_fails_activate` into `master`