Add error message for duration subtraction overflow in sim tests (#6558)

* Add error message for duration subtraction overflow.

testing/simulator/src/basic_sim.rs

@@ -206,7 +206,7 @@ pub fn run_basic_sim(matches: &ArgMatches) -> Result<(), String> {
             node.server.all_payloads_valid();
         });
 
-        let duration_to_genesis = network.duration_to_genesis().await;
+        let duration_to_genesis = network.duration_to_genesis().await?;
         println!("Duration to genesis: {}", duration_to_genesis.as_secs());
         sleep(duration_to_genesis).await;
 

testing/simulator/src/fallback_sim.rs

@@ -194,7 +194,7 @@ pub fn run_fallback_sim(matches: &ArgMatches) -> Result<(), String> {
             );
         }
 
-        let duration_to_genesis = network.duration_to_genesis().await;
+        let duration_to_genesis = network.duration_to_genesis().await?;
         println!("Duration to genesis: {}", duration_to_genesis.as_secs());
         sleep(duration_to_genesis).await;
 

testing/simulator/src/local_network.rs

@@ -459,7 +459,7 @@ impl<E: EthSpec> LocalNetwork<E> {
             .map(|body| body.unwrap().data.finalized.epoch)
     }
 
-    pub async fn duration_to_genesis(&self) -> Duration {
+    pub async fn duration_to_genesis(&self) -> Result<Duration, &'static str> {
         let nodes = self.remote_nodes().expect("Failed to get remote nodes");
         let bootnode = nodes.first().expect("Should contain bootnode");
         let now = SystemTime::now().duration_since(UNIX_EPOCH).unwrap();
@@ -471,6 +471,9 @@ impl<E: EthSpec> LocalNetwork<E> {
                 .data
                 .genesis_time,
         );
-        genesis_time - now
+        genesis_time.checked_sub(now).ok_or(
+            "The genesis time has already passed since all nodes started. The node startup time \
+            may have regressed, and the current `GENESIS_DELAY` is no longer sufficient.",
+        )
     }
 }