Print meaningful error message if the user/group providers are not suitable when applying the settings catalog

Hi Folks

We have a problem when trying to run the Puppet Agent on a freshly reinstalled Mac. This issue is occurring on most versions on MacOS - Sierra, High Sierra, Mojave

What is happening, is that after the first Puppet run, Puppet will fail to run again and throw the following error. If we delete the cache in /opt/puppetlabs/puppet/cache, we can re-run Puppet again, however the error will then re-occur on the next run, and the cache will need deleting again.

Error: Could not create resources for managing Puppet's files and directories in sections [:main, :agent, :ssl]: undefined method `exists?' for nil:NilClass

Error: Could not prepare for execution: Could not create resources for managing Puppet's files and directories in sections [:main, :agent, :ssl]: undefined method `exists?' for nil:NilClass

undefined method `exists?' for nil:NilClass

This also happens in no-op mode, so I speculate it's something to do with merely downloading the cache files from the server.

I'd like to stress that we're only seeing this on freshly reinstalled Macs (using Apple's internet recovery). Macs new from the shop are fine.

Thanks

JT