message: Lint error actually pointed at type mismatch
  Strange tsc didn't catch this.
diff: @@ -80,7 +80,7 @@ export class Handler implements DBHandler {
  protected readonly writeLock = new Mutex();
  protected readonly db: LevelUp;
  - constructor(protected readonly dbPath: string, errorCallback?: (err: Error) => any) {
  + constructor(protected readonly dbPath: string, errorCallback?: (err: Error | undefined) => any) {
  this.db = new LevelUpCtor(new LevelDown(dbPath), errorCallback);
  }
  