Better error resilience.

[squidarth]

Right now, truss predict behaves weirdly if there are errors. This makes us more resilient to those errors. The change in this PR defends against this case:

$ [~]$ curl -X POST https://app.baseten.co/models/9P2boVP/predict \
  -H 'Authorization: Api-Key g9RlcBM6.JfudVBlNqMjTNbnprEQAWHoZtbUajWrh' \
  -d '{"a": "b", "c": true}'
{
	"error": "Model is not ready, it is still building or deploying"
}

specifically

[joostinyi]

might be more clear to check for existence of the error key if we know it will always come with that