Use response inside try block in computer.ai fast_llm

[liangjs]

Describe the changes you have made:

The response object will not be available if an exception is thrown when executing llm.interpreter.chat, so we should only access the response inside the try block.

Pre-Submission Checklist (optional but appreciated):

• I have included relevant documentation updates (stored in /docs)
• I have read docs/CONTRIBUTING.md
• I have read docs/ROADMAP.md