Error_message is a special global. It's set when the app (Current_app = 'run') encounters an error and switches to the source editor, and cleared when switching from source editor back to the app.
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if Error_message then
local height = math.min(20*Editor_state.line_height, App.screen.height*0.2)
App.color{r=0.8,g=0,b=0}
love.graphics.rectangle('fill', 150, App.screen.height - height-10, App.screen.width, height+10)
App.color{r=0,g=0,b=0}
love.graphics.print(Error_message, 150+10, App.screen.height - height)
end