Show error toast on termination PythonError
When a Python error is raised when terminating a run, the "Terminate" button doesn't appear to do anything and no error is surfaced.
Resolve this by ensuring that the PythonError is also handled in the mutation.
Test Plan: Run an intentionally slow pipeline, with a raise Exception('oh no') in the termination codepath. Click "Terminate", verify that the error toast appears.
Reviewers: catherinewu, bengotow, prha
Reviewed By: prha
Differential Revision: https://dagster.phacility.com/D4452