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

[JasonWeill]

Problem

As noted in #789 (comment), 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.