Improve error messages

[Martoon-00]

Clarification and motivation

I didn't care much about error messages till now, and there are multiple things that deserve attention:

• In full interpolator, in some cases invalid syntax produces very weird errors;
• The message when ? is supplied could be prettier;
• The source locations reported by the parser could be not quoter-relative but global, see this section of our post on parser combinators;
• When failing switch is supplied (examples: [int|S||], [int|?||]), the error message points not to the switch, but to the next character after the switch. And I found it tricky to resolve back then 🤔

Acceptance criteria

• The mentioned error messages are fixed;
• Other error scenarios are tried, the error messages are improved there too if necessary.