Implement Exception Handling by Graphene Middleware

[jopemachine]

Main idea

In the current code, each mutation requires manually injecting ok and msg in the form of cls(ok="...", msg="...") every time.
This approach is prone to mistakes, and since it involves code related to the API interface, mistakes can be difficult to roll back.

We can refactor it to simply throw an error when an error occurs, allowing the middleware to handle it instead.
It seems this middleware could be added at the following location.

backend.ai/src/ai/backend/manager/api/admin.py

Lines 97 to 101 in eadddf6

	middleware=[
	GQLLoggingMiddleware(),
	GQLMutationUnfrozenRequiredMiddleware(),
	GQLMutationPrivilegeCheckMiddleware(),
	],