[backport#17381 5/5] Add missing SetupGeneration error handling in EncryptWallet

Summary:
Suggested https://github.com/bitcoin/bitcoin/pull/17304#discussion_r341286026
by me

https://github.com/bitcoin/bitcoin/pull/17381/commits/05b224a175065aee4d6d9c471722bc4503f01fdf

Depends on D7419

Concludes backport of Core PR17381

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7420