Function user_message 

pub fn user_message(err: &Error) -> String

A message for the people an embedding host serves: the innermost cause without Lua’s stack traceback: block. A host function’s Err(e) surfaces as e’s own text; a Lua error("msg") surfaces as file:line: msg.

sgen: content/no-date.md: front matter: 'date' is required

instead of that line followed by stack traceback: [C]: in method 'pages' ....

This is the answer for a program whose users did not write the Teal and cannot act on its frames — a static site generator telling an author which file is missing a date. It is not the answer for whoever is developing the program: see developer_message, which is what htl run and htl test print.