message: Remove .toString() from disk error handling
  For the most part, error messages look like:
  ```js
  { error:
  { errno: -2,
  code: 'ENOENT',
  syscall: 'stat',
  path: '/data/path/stuff/place' } }
  ```
  But `toString()` just turns it into `[object Object]`, which is quite useless as a log message.
diff: @@ -116,7 +116,7 @@ export class DiskMangerWorker {
  return resolve(directory);
  } catch (err) {
  - return reject({error: err.toString()});
  + return reject({error: err});
  }
  });
  