feat: Display Meaningful Error Message When Path is Missing

[pahund]

If you try to render ads without an ad unit path being present either in the global config or slot config, react-prebid will happily try to initialize GPT slots, prompting an error from the gpt.js library, e.g.:

Exception in queued GPT command TypeError: "slot is null"

Validate if an ad unit path is provided and give the developers a useful error message in this case.