Merge #12298: Refactor HaveKeys to early return on false result

Summary:
5bdbbdc Refactor HaveKeys to early return on false result (João Barbosa)

Pull request description:

This consists in a trivial change where the return type of `HaveKeys()` is now `bool` meaning that it returns whether all keys are in the keystore, and early returns when one isn't.

Tree-SHA512: 03e35ea8486404b84884b49f6905c9f4fc161a3eeef080b06482d77985d5242a2bdd57a34b8d16abe19ee8c6cfa3e6fbcb935c73197d53f4cd468a2c7c0b889b

Backport of Core PR12298

https://github.com/bitcoin/bitcoin/pull/12298/files

Test Plan:

make check
test_runner.py wallet_*

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3657