Show rendering errors to user

This also delays the response to the HTTP request until the printer has
printed whatever it should.

showbits-thermal-printer/src/documents/cells/mod.rs

@@ -146,6 +146,5 @@ pub async fn post(server: State<Server>, Form(form): Form<FormData>) -> somehow:
         .with_file("/image.png", bytes)
         .with_main_file(include_str!("main.typ"));
 
-    server.print_typst(typst).await;
-    Ok(())
+    server.print_typst(typst).await
 }