Use user-/idiot-friendly error messages in `settings` command

[lionkor]

Currently, mistyping settings could confuse users who mistype settings.

Acceptance criteria

• [ ] Use simple language to convey what the error is, for example "unknown setting", "unknown category" (or similar) when users use an invalid category or setting.
• [ ] Hint at possible errors using language such as "Maybe it was spelled wrong", "use settings list to get a list of all valid settings", "setting names are case sensitive", bla.

Notes:

• It's probably easiest to swap logic/runtime error types for custom exception types and then catch them via a } catch (InvalidCategory e) {...} catch (InvalidKey e) { ... } ... etc.
• You can write a grammar for the settings, using e.g. boost::spirit