Issue warnings when the configuration has unrecognised fields

[f-f]

As noted e.g. in #1159, the fact that we just ignore fields that we don't parse means that it's possible to get the names of the fields wrong and not know about it.

I am not sure how the fix for this one would look like - I think either we'll end up doing some low-level parsing with the yaml library in Config.js, or hopefully we can do something in the Codec at the PS level