message: Nicer error messages when missing filesystem.
diff: @@ -92,8 +92,12 @@ Status Env::GetFileSystemForFile(const string& fname, FileSystem** result) {
  io::ParseURI(fname, &scheme, &host, &path);
  FileSystem* file_system = file_system_registry_->Lookup(scheme.ToString());
  if (!file_system) {
  - return errors::Unimplemented("File system scheme ", scheme,
  - " not implemented");
  + if (scheme.empty()) {
  + scheme = "[local]";
  + }
  +
  + return errors::Unimplemented("File system scheme '", scheme,
  + "' not implemented (file: '", fname, "')");
  }
  *result = file_system;
  return Status::OK();
  @@ -173,8 +177,8 @@ bool Env::FilesExist(const std::vector<string>& files,
  if (!file_system) {
  fs_result = false;
  if (fs_status) {
  - Status s = errors::Unimplemented("File system scheme ", itr.first,
  - " not implemented");
  + Status s = errors::Unimplemented("File system scheme '", itr.first,
  + "' not implemented");
  local_status.resize(itr.second.size(), s);
  }
  } else {
  