Errors in contacting model show up with code mini-toolbar in chat UI

[JasonWeill]

trafficstars

Problem

As noted in https://github.com/jupyterlab/jupyter-ai/pull/789#issuecomment-2120858017, all of the output of the chosen model in the chat UI is rendered in markdown, even if it represents an error message. This causes the code mini-toolbar added in #789 to show up when there's a failure in contacting the model, even though it represents an error message, not code.

Proposed Solution

Use a different data model and DOM model for error reporting when contacting a model.