validation: report if pruning prevents completion of verification

Summary:
Now the verifychain RPC returns false if the checks didn't
finish because the blocks requested to be queried have been pruned.

This is a partial backport of core#25574
https://github.com/bitcoin/bitcoin/pull/25574/commits/57ef2a4812f443b2d734f43cebf3ef5038da83f2

Depends on D14928

Test Plan: ninja && ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14929