Tweak prompt for length and no relevant results found

[kdid]

Summary

• Instructs prompt to keep reading time of answers under 45 seconds
• Instructs prompt to suggest searching NUL's catalog if no relevant results are found in DC
• Handles error flow where response.py's prepare_response was constructing and returning an error object but chat.py was not accounting for the error shape. (Example: https://github.com/nulib/repodev_planning_and_docs/issues/5071)
  • Now error will be logged in default lambda log (not metrics) and a generic Internal Server Error returned