Connection Error in main window could use a scrollbar

[philrz]

Repro is with Zui commit a1cc8fc.

It's only when I was doing final verification of the changes linked to #1497 that I realized that at the default window size, when the Connection Error appears in the main window, the links to Retry and View Troubleshooting Docs are out of view at the bottom of the window, and there's no scrollbar to provide a visual cue that there's anything down there to see.

In a related topic, I'm reminded that #2620 spoke of a similar problem when a tall backend panic error was displayed in the main window. So perhaps we could address that at the same time.

See the attached video for repro.

Repro.mp4