Smarter feedback for setup strings

[Intery]

Idea

Current feedback given an un-parseable setup string is simply "setup string not understood".
Provide feedback at least containing

• The section which triggered the error,
• The particular error in the section,
• Short examples of valid setup strings,
• A reference to the setup string guide (when it exists).

For some errors, it is also feasible to generate a "Do you mean?" suggestion.

[Intery]

Partially implemented in the 9634af1 rewrite