Fix UnboundLocalError in _DownloadObjectToFileResumable
Fix UnboundLocalError in _DownloadObjectToFileResumable

This fixes a bug where opening the download file for read
to determine its current position could fail, and the
subsequent attempt to close the file would cause an
UnboundLocalError.

GoogleCloudPlatform/gsutil,GoogleCloudPlatform/gsutil,BrandonY/gsutil,fishjord/gsutil