Make verify_deployable_apps and check_puppet_names abort on error
Make verify_deployable_apps and check_puppet_names abort on error

Prior to this change, they only outputted as strings (which in the
check_puppet_names case was not a very useful output, as it would
print out the full HTML source of a 404ing page).
Aborting properly means we can hook these tasks into build
pipelines.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs