Errors now show as toasts in the bottom right corner

web/CODE_CONVENTIONS.md

@@ -299,6 +299,10 @@ backend.
   quotes and apostrophes are both fine.
 - Errors are shown where the user is looking, with `role="alert"`, and can
   be retried or dismissed; they're never only logged to the console.
+- An action that failed (a mark that wasn't saved) is shown as a toast in
+  the bottom right corner (`createToasts`, `ToastList`), which goes after
+  15 seconds or when it's dismissed. A load that failed is shown in place of what didn't
+  load, with a way to retry, and as a toast too.
 
 ## Security