wallet: fix importdescriptor silent fail

Summary:
Currently importdescriptor command will successfully import a descriptor with hardened derivations into a watch-only wallet while silently failing to expand the descriptor to fill the cache. This leads to a broken wallet state and failure to load such wallet due to missing cache on subsequent restart.

This is a backport of core#20153

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10721