Improve error handling

[imobachgs]

• An attempt to address some of the issues reported in Miscellaneous problems with agama config set #688.
• Fix handling of users/root settings: the CLI ignored the settings but did not report any problem. This bug was introduced in Better Settings derive macro #666. I needed to extend the derive macro to support flatten (in serde jargon) fields.
• Extend the agama_settings documentation to explain better how it works.

Click for a documentation screenshot 😉

[coveralls]

coverage: 72.098% (-0.06%) from 72.158% when pulling a81fd69 on imobachgs:improve-error-handling into 9c7a6ad on openSUSE:master.

[mvidner]

I can't figure out which problem is solved by c7fed77 "Fix the detection of unknown attributes from config set/add"

[mvidner]

OK, this now works:

agama config set user.user_name=tux user.password=foo
agama config set root.password=foo
agama config set root.sshPublicKey=Roooot

[imobachgs]

I can't figure out which problem is solved by c7fed77 "Fix the detection of unknown attributes from config set/add"

In some cases, the old code implemented 1-line methods that just returned Ok(()). Now I have fixed the condition to determine when to implement a method.

[imobachgs]

I would recommend generating the documentation for agama_settings with cargo doc --no-deps --package agama-settings --open.

[teclator]

LGTM