code: def prepare(self):
          
          for s in self.get_all_sources():
              if s.missing():
                  msg = "Source `%s' not found, needed by target `%s'."
                  raise SCons.Errors.StopError(msg % (s, self.batches[0].targets[0]))
docs: Preparatory checks for whether this Executor can go ahead
          and (try to) build its targets.