Provides a more useful error message when users try to use this class by itself. Without this fix, you'll get something like
```
Error: Evaluation Error: Operator '[]' is not applicable to an Undef Value. at /etc/puppetlabs/code/environments/production/modules/apt/manifests/update.pp:7:8"?
```
class apt::update {
+ assert_private
+
#TODO: to catch if $::apt_update_last_success has the value of -1 here. If we
#opt to do this, a info/warn would likely be all you'd need likely to happen
#on the first run, but if it's not run in awhile something is likely borked