ModelCard init breaks if model-index exists but is invalid

[nateraw]

For example, in this repo there is no dataset defined, so it breaks:

from modelcards import ModelCard

card = ModelCard.load('nateraw/rare-puppers')

It would be nice if it tried to partial load this, then helped users mitigate issues with validation.

As a quick fix, I think I'll just ignore model-index in these cases for now, logging a warning to give an indication of what's going on.