Reworded entrypoint scripts error messages to avoid confusion

mainline/alpine-perl/10-listen-on-ipv6-by-default.sh

mainline/alpine-perl/20-envsubst-on-templates.sh

mainline/alpine/10-listen-on-ipv6-by-default.sh

mainline/alpine/20-envsubst-on-templates.sh

mainline/buster-perl/10-listen-on-ipv6-by-default.sh

mainline/buster-perl/20-envsubst-on-templates.sh

mainline/buster/10-listen-on-ipv6-by-default.sh

mainline/buster/20-envsubst-on-templates.sh

stable/alpine-perl/10-listen-on-ipv6-by-default.sh

stable/alpine-perl/20-envsubst-on-templates.sh

stable/alpine/10-listen-on-ipv6-by-default.sh

stable/alpine/20-envsubst-on-templates.sh

stable/buster-perl/10-listen-on-ipv6-by-default.sh

stable/buster-perl/20-envsubst-on-templates.sh

stable/buster/10-listen-on-ipv6-by-default.sh

stable/buster/20-envsubst-on-templates.sh