Function format_check_errors_plain 

pub fn format_check_errors_plain(
    db: &impl Db,
    result: &CheckResult,
    file: TextFile,
    cst: &Cst,
    origins: &OriginMap,
) -> Result<String, QueryError>

Format check errors without ANSI colors (for testing).

Returns Err with suspension if file content isn’t loaded yet.