index: Avoid async shutdown on init error
Summary:
An async shutdown during init is confusing when a simple boolean return value can be used for a synchronous shutdown.
This is a backport of core#21796
Depends on D13911
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D13912