[dagit] Show syntax errors in config editor

Summary:
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

Test Plan:
before:

after:

Reviewers: #ft, max

Reviewed By: #ft, max

Subscribers: max

Differential Revision: https://dagster.phacility.com/D656