Function error_overlay_html_in 

pub fn error_overlay_html_in(
    err: &ParseError,
    source: &str,
    reload_client: &str,
    file: Option<&str>,
) -> String

The same overlay, told WHICH FILE it is drawing.

A dev server serves many pages and each may import several modules, so “line 24” on its own sends a reader to look in whichever file they had open. The name costs one line and removes the guess.