Changed signurl command check readability function to use storage_api.httpwrapper.MakeRequest which retries on 5xx errors
Changed signurl command check readability function to use storage_api.httpwrapper.MakeRequest which retries on 5xx errors

dimfeld/gsutil,chriskuehl/gsutil-debian,GoogleCloudPlatform/gsutil,fishjord/gsutil,BrandonY/gsutil,craigcitro/gsutil,ttiurani/gsutil,mattdr/gsutil,dtjackson/gsutil,GoogleCloudPlatform/gsutil