[dagit] Improve PythonError case in RunGroupPanel

Summary: Clean up the PythonError case in RunGroupPanel, which is currently just some text jammed into the UI.

Test Plan: Force error codepath, verify proper rendering and error dialog behavior.

Reviewers: nate, prha, sashank

Reviewed By: sashank

Differential Revision: https://dagster.phacility.com/D5998