message: Throw when mmap_reserve() fails
diff: @@ -371,6 +371,9 @@ void* mmap_reserve(FileDesc fd, size_t reservation_size, size_t offset_in_file)
  return nullptr;
  #else
  auto addr = ::mmap(0, reservation_size, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
  + if (addr == MAP_FAILED) {
  + throw std::runtime_error(get_errno_msg("mmap() failed: ", errno));
  + }
  return addr;
  #endif
  }
  