message: improve counterexample printing of UB fn calls
diff: @@ -212,6 +212,9 @@ static bool error(Errors &errs, const State &src_state, const State &tgt_state,
  if (m.eval(val.return_domain).isFalse()) {
  s << *var << " = function did not return!\n";
  break;
  + } else if (m.eval(val.domain).isFalse()) {
  + s << "Function " << call->getFnName() << " triggered UB\n";
  + continue;
  } else if (var->isVoid()) {
  s << "Function " << call->getFnName() << " returned\n";
  continue;
  