improve error message for attempting to add systems using add_system_…

…to_stage

Fixes #3250

Notes:
Since this panic occurs in bevy_ecs, and StartupStage is part of
bevy_app, we really only have access to the Debug string of the
`stage_label` parameter.  This led me to the hacky solution of
comparing the debug output of the label the user provides with the known
variants of StartupStage.

An alternative would be to do this error handling further up in
bevy_app, where we can access StartupStage's typeid, but I don't think
it is worth having a panic in 2 places (_ecs, and _app).

crates/bevy_ecs/src/schedule/mod.rs

@@ -206,6 +206,13 @@ impl Schedule {
         // of the game. Closures inherit generic parameters from their enclosing function.
         #[cold]
         fn stage_not_found(stage_label: &dyn Debug) -> ! {
+            if ["PreStartup", "Startup", "PostStartup"]
+                .contains(&format!("{:?}", stage_label).as_str())
+            {
+                panic!(
+                    "It looks like you may be trying to add a startup system. Use `add_startup_system_to_stage()` instead of `add_system_to_stage()`.",
+                )
+            }
             panic!(
                 "Stage '{:?}' does not exist or is not a SystemStage",
                 stage_label