[wallet] Close DB on error.

Summary:
[wallet] Close DB on error.

Backport of Core PR 11017
https://github.com/bitcoin/bitcoin/pull/11017

Completes T606

Test Plan:
make check
test_runner.py
Sanity check open and close (both by exiting and by ctrl-C interrupt) bitcoin-qt

Reviewers: jasonbcox, deadalnix, Fabien, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2807