When running with `--config=diff`, produce an error when the configuration file is invalid

[fredmorcos]

• Program: Authoritative, Recursor
• Issue type: Bug report

Short description

When running with --config=diff, produce an error when the configuration file is invalid.

Environment

• Operating system: N/A
• Software version: -git
• Software source: PowerDNS repository

Steps to reproduce

Using the auth, as an example, with the following configuration file:

local-address=0.0.0.0,::
foo=yes

and running pdns_server --config=diff --config-dir=., the output is the diff without an error message stating that foo is an invalid configuration key:

# Autogenerated configuration file based on running instance (2022-09-09 13:50:22 +0200)

config-dir=.
domain-metadata-cache-ttl=60
local-address=0.0.0.0,::

Expected behaviour

The expected output should be an error message like the following (taken from running with --config=check):

Fatal error: Trying to set unknown setting 'foo'

Actual behaviour

Got the diff output while using an erroneous configuration file.

Other information

For the time being, the recursor also behaves the same way for consistency.