Merge #14813: qa: Add wallet_encryption error tests

Summary:
fa739d4bd7 qa: Add wallet_encryption error tests (MarcoFalke)

Pull request description:

The errors for empty passphrases are the help text of the RPC call, which is not very specific. Replace that with proper RPC errors and test them.

Tree-SHA512: 3137e0f8f2e42a1f8ab1eeb57c99052557725f6f85139ff48c24acc8f3cf4087802de5216f3ce97375b291d21bddb7cd1379a6f280166136a306a0c9663bbd42

Backport of Core PR14813

Test Plan:

ninja
test_runner.py wallet_encryption

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5582