Update error messages that still refer to `puppet cert`

A recent user report uncovered at least one place in Puppet's error output where we are still referring to the removed puppet cert commandL https://github.com/puppetlabs/puppet/blob/edc2d4c0ffb2b60879e61f232f62c7fd6927733d/lib/puppet/ssl/host.rb#L162. We should fix that, and also audit the code base for other bad references.