fix src tarball error text in make-release

Correct the error message if make-release fails to create the source
tarball.

Change-Id: Iaf908975d56c0ee4c2959854f0ffd9c855f2c708
Reviewed-on: http://gerrit.openafs.org/10228
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>