Cody: Persist error messages

[abeatrix]

RE: #51759

Issue: error displaying as chat message will disappear all of a sudden without user action

I noticed in the current implementation, errors resulted from failed streaming will replace the current assistant message, which would include the previous error message, or things that Cody has already response. This PR includes a fix where we will add the error message ON TOP of the current assistant message instead of overriding it.

For example, instead of removing what Cody has replied and replace it with an error message, we now add the error message after what Cody has already answered Hello! How can I help you?:

Other changes include:

• Added a new <ErrorBanner> component that would show 5 of the last system error messages as a banner in chat UI (instead of 1, which override the previous one).
• Now log all system error messages to debug logs ("cody.debug": true) view in UI

Test plan

See screenshots above

[sourcegraph-buildkite]

Bundle size report 📦

Initial size	Total size	Async size	Modules
0.00% (0.00 kb)	0.00% (+0.14 kb)	0.00% (+0.14 kb)	0.00% (0)

Look at the Statoscope report for a full comparison between the commits 6faf323 and a9917a7 or learn more.

Open explanation

• Initial size is the size of the initial bundle (the one that is loaded when you open the page)
• Total size is the size of the initial bundle + all the async loaded chunks
• Async size is the size of all the async loaded chunks
• Modules is the number of modules in the initial bundle

[sourcegraph-bot]

📖 Storybook live preview

[philipp-spiess]

Nice! LGTM. I'll merge this right away so I can base some of the other fixes on top of it. Thanks so much!