Don't ignore error from zip buildpack download
Don't ignore error from zip buildpack download

The error returned by zipDownloader was only scoped to the IsZipFile
block. So any error it would returned would be ignored and
downloadBuildpacks would return success

Signed-off-by: Sunjay Bhatia <df2d57be7987a471915f625a5499155bc16d4200@pivotal.io>

cloudfoundry-incubator/buildpack_app_lifecycle,cloudfoundry-incubator/buildpack_app_lifecycle