message: Fix build with exceptions disabled
diff: namespace caf::test {
  +#ifdef CAF_ENABLE_EXCEPTIONS
  +
  requirement_error::requirement_error(std::string msg) : what_(std::move(msg)) {
  // nop
  }
  @@ -37,6 +39,8 @@ const char* requirement_error::what() const noexcept {
  return what_.c_str();
  }
  +#endif // CAF_ENABLE_EXCEPTIONS
  +
  class watchdog {
  public:
  static void start(int secs);
  