Output client-side warning when govuk_class unset
Output client-side warning when govuk_class unset

Generate a client-side notification in addition to the server-side warning
when `FACTER_govuk_class` is not set. Including some additional advice about
the possible cause. This will make it obvious when a new node's first agent
run might have been exceedingly fast and ineffective.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet