[dagit] Surface errors on backfill mutation
When errors are returned from GraphQL when kicking off a backfill, we currently swallow the errors. Instead, display them more aggressively.
This isn't a great long term solution, but we also don't have really good ways of consistently surfacing stack traces in Dagit UI. It's certainly better than nothing though.
Test Plan: Create a schedule that uses an invalid mode, try to kick off a backfill for it. Verify that the error appears.
Reviewers: catherinewu, sandyryza
Reviewed By: catherinewu
Differential Revision: https://dagster.phacility.com/D5330