make CommandError non_exhaustive

Author: NathanSWard

crates/bevy_ecs/src/system/commands/fallible.rs

@@ -5,6 +5,7 @@ use std::fmt::Debug;
 
 /// The error struct provided to command error handlers.
 /// This contains both the error, and a mutable reference to [`World`].
+#[non_exhaustive]
 pub struct CommandError<'a, C: FallibleCommand> {
     pub error: C::Error,
     pub world: &'a mut World,