Pass-through HttpError caught in multipart handler (#867)

- Consumers of express-openapi-validator have access to the custom error
  types via exported object: error (e.g. error.BadRequest).
- If the multipart handler throws, for example from the multer storage
  engine, check whether the err instance is already an HttpError. If so,
  it can be passed-through as is. This is mostly useful for setting the
  HTTP status code.

src/middlewares/openapi.multipart.ts

@@ -127,6 +127,8 @@ function error(req: OpenApiRequest, err: Error): ValidationError {
       : !unexpected
       ? new BadRequest({ path: req.path, message: err.message })
       : new InternalServerError({ path: req.path, message: err.message });*/
+  } else if (err instanceof HttpError) {
+    return err;
   } else {
     // HACK
     // TODO improve multer error handling