Consolidates configuration error handling by moving the error type and
logic from assistant_context_editor to language_model::registry.
The registry now provides a single method to check for configuration
errors, making the error handling more consistent across the agent panel
and context editor.
This also now checks if the issue is that we don't have any providers,
or if we just can't find the model.
Previously, an incorrect model name showed up as having no providers,
which is very confusing.
Release Notes:
- N/A
Ben Brandt
9427833fdf