Improve error message when partner tries to manually update/create an OWNERS file

[mgoerens]

Currently the error message makes it feels like creating or updating an OWNERS file manually via PR is possible, just requires a review from the maintainers. This is not the case and this is confusing, leading to us having to explain that they need to go through Connect.

Example: openshift-helm-charts/charts#2038

[mgoerens]

This should be already fixed by #450 which hasn't been rolled out to prod just yet.