Improve agent exception handling process.

[LittleLittleCloud]

Current behavior

When an agent raises an exception during the response, the UI doesn't present an error message to the user. Instead, it remains in the generating status indefinitly.

What to do

• Implement error notification by adding a status indicator to inform users when an agent encounters an exception during response. Stop orchestration when GenerateReply returns a bad request, and show the error message in toaster #151
• Resolve stuck status by recovering from generating status to responsive status. Stop orchestration when GenerateReply returns a bad request, and show the error message in toaster #151