Better error reporting

[bderenzi]

When there are issues with API keys and provider settings, the user just sees "this is probably temporary" and doesn't get any feedback that they should check their API key. I think there are a few other situations where this comes up. e.g., I think this happens if we have an experiment that is tied to a LLM then you edit the provider and remove the model.

[SmittieC]

This is similar to #449