message: don't crash on missing file when deleting it
  if the file does not exists do not crash when trying to delete it.
diff: @@ -211,8 +211,10 @@ void GlobalNotifier::Impl::release_version(sync::ObjectID id, VersionID old_vers
  if (info.coordinator) {
  std::string path = info.coordinator->get_config().path;
  m_realms.erase(it);
  + if (File::exists(path)) {
  File::remove(path);
  }
  + }
  else {
  m_realms.erase(it);
  }
  