Get rid of NoValue. More use of Errors in parser

[mneumann]

Clean up the code to use proper parse errors, also including the filename/line number. Don't return NoValue in case of an error, instead return a proper Error. Maybe also differentiate between a SyntaxError and a SemanticError (e.g. duplicate section).

Also make use of unreachable!() macro instead of assert!(false) in some places.