[backport#15744] refactor: Extract ParseDescriptorRange

Summary:
510c6532ba Extract ParseDescriptorRange (Ben Woosley)

Pull request description:

So as to be consistently informative when the checks fail, and
to protect against unintentional divergence among the checks.

Depends on D6614

Backport of Core PR15744

Test Plan:

ninja check
test_runner.py wallet_importmulti rpc_scantxoutset

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6615