(PUP-3244) ENC ignores missing directory environments

When using an ENC, if an environment that didn't exist was defined for a
node puppet would place the node into the production environment instead
of raising an error to alert the user that the environment that had been
requested did not exist.

This commit changes the behavior when an environment is defined in an
ENC but does not actually exist. Instead of placing the node into the
production environment a Puppet::Environments::EnvironmentNotFound error
is raised.
I think Cached.get! would still work appropriately without the code there as it should call the get method of Cached and not Combined. Checking now.