In case `wget` is not installed, downloading the Helm release will fail.
The `install-helm.sh` script won't return a fatal error in that case,
and CI jobs continue running in an environment that is not ready.
By adding a check that exist the script with a failure, the CI will now
correctly report a problem when Helm can not be downloaded.
See-also: #3430
Signed-off-by: Niels de Vos <ndevos@redhat.com>