message: InvalidGrant should be used for unknown or expired refresh tokens.
diff: @@ -4,7 +4,7 @@ import crypto from 'crypto';
  import db from '../database';
  import * as UserService from '../user/service';
  import { User } from '../user/types';
  -import { InvalidRequest, UnauthorizedClient } from './errors';
  +import { InvalidRequest, UnauthorizedClient, InvalidGrant} from './errors';
  import { OAuth2Client, OAuth2Code, OAuth2Token } from './types';
  // 10 minutes
  @@ -172,7 +172,7 @@ export async function generateTokenFromRefreshToken(client: OAuth2Client, refres
  } catch (err) {
  if (err instanceof NotFound) {
  - throw new InvalidRequest('The refresh token was not recognized');
  + throw new InvalidGrant('The refresh token was not recognized');
  } else {
  throw err;
  }
  