Bugfixes for dagits checking cookies on each request

Summary:
results of a debugging session with dish:

• better error visualzation when search bootstrap query fails (at least if you're looking at the response in devtools)
• fix bad typehint that that query was hitting

Test Plan: load dagit with cookie validation enabled

Reviewers: dish

Reviewed By: dish

Differential Revision: https://dagster.phacility.com/D8467