[FR][DAC] raise a better exception for missing contents in config files

[brokensound77]

Note: this work will target the DAC-feature branch

related to #3407

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As part of #3407, the init setup new files, but blank. If a user runs tests or the loader before populating them, an error will raise (such as for missing info in the new packages.yml file). However, it will not be obvious as to why. We should wrap this to be more explanatory as to why the error is raising.

A candidate option would be to do it here:

https://github.com/elastic/detection-rules/blob/DAC-feature/detection_rules/config.py#L198

and assert that any required contents are populated