[dagit] Show syntax errors in config editor
Fixes issue in: https://github.com/dagster-io/dagster/issues/1571
When entering config in dagit, if you enter config that is not a dict, the validation error is caught at the wrong layer and is displayed as a "GraphQL Error".
Instead, we can check if the config is not a dict client side and display a syntax error
Reviewers: #ft, max
Reviewed By: #ft, max
Differential Revision: https://dagster.phacility.com/D656