Don't say "No stack provided" for errors with no stack
Summary: There are valid cases to show an error with no stack (e.g. a config issue). Drop the extra verbiage.
Launch a backfill with a config error, bask in the glow of a nice clean error dialog
Launch a backfill with a system error, see the stack trace still
Reviewers: dish, bengotow
Reviewed By: dish
Differential Revision: https://dagster.phacility.com/D5462