Improve error handling for slash commands

What does this merge request do and why?

This MR improves error handling for Slash commands.

• Parse errors payload from GraphQL.
• Handle schema fetching error.
• Add debug log to facilitate troubleshooting.

How to set up and validate locally

1. Ensure GCP environment variables are setup.
2. Check out to this merge request's branch.
3. Run the follow command to kick off the pipeline.

   poetry run promptlib duo-chat eval --config-file=data/config/root_cause_analysis/eval_slash_troubleshoot_experiment.example.json --test-run --sample-size 1

Merge request checklist

• I've ran the affected pipeline(s) to validate that nothing is broken.
• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.