Function install_undefined_error_constructor 

pub fn install_undefined_error_constructor(token: Token, content: &str)

Install a Constructor for token whose sole effect at digestion time is to emit <ltx:ERROR class='undefined'>content</ltx:ERROR> (the Rust equivalent of Perl Document::makeError). It logs NOTHING — the caller is responsible for the Error!/note_status. Mirrors the make_error constructor that generate_error_stub installs for undefined commands, so undefined environments (\begin{undefinedenv}) leave the same visible <ltx:ERROR> marker as Perl instead of silently vanishing from the output.