[backport#15629] init: Throw error when network specific config is ignored

Summary:
fae38c3dc6 doc: Fix all typos reported by codespell (MarcoFalke)
fa9058f0ed doc: Add release notes for 15629 (MarcoFalke)
fa4a922d78 qa: Add test for missing testnet section in conf file (MarcoFalke)
dddd6f0f58 init: Throw error when network specific config is ignored (MarcoFalke)

Pull request description:

This should have no effect on mainnet users, but simplifies testing, where config settings are currently ignored with only a warning. Fix this by making it an error.

Issues:
*  bitcoin client 0.17.0 ignores wallet's name (file) #14523
*  Can't set custom rpcport on testnet #13777
* ...

ACKs for commit fae38c:

Tree-SHA512: 2e209526898eea6e444c803ec2666989cee4ca137492d32984998733c50a70056cb54657df8dc3027a6a0612738a8afce0bc35824b868c5f22281e00e0188530

Depends on D5758

This is a backport of Core PR15629

Test Plan:

ninja check
test_runner.py feature_config_args.py

Reviewers: deadalnix, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5759