This way, the Travis job will fail, and ci_manifest.sh will not
continue to upload a manifest for an untagged-* tag.

Related to https://github.com/w3c/web-platform-tests/issues/10572 but
doesn't fix it, rather it'll make the Travis jobs fail when this
problem happens, making it easier to notice.

Drive-by: use the sha instead of "master" in log output