Diagnose unsuitable outputs in lockunspent().

Summary:

Fixes "lockunspent returns true even for non-existent outputs"

Backport of core PR11087
https://github.com/bitcoin/bitcoin/pull/11087/files

Test Plan:

make check
./test/functional/test_runner.py wallet_basic

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3420