feat: better error message for light_client/bootstrap endpoint (#7597)

Fixes #7229

Author: PoulavBhowmick03

beacon_node/beacon_chain/src/light_client_server_cache.rs

@@ -374,7 +374,7 @@ impl<T: BeaconChainTypes> LightClientServerCache<T> {
         let Some(current_sync_committee_branch) = store.get_sync_committee_branch(block_root)?
         else {
             return Err(BeaconChainError::LightClientBootstrapError(format!(
-                "Sync committee branch for block root {:?} not found",
+                "Sync committee branch for block root {:?} not found. This typically occurs when the block is not a finalized checkpoint. Light client bootstrap is only supported for finalized checkpoint block roots.",
                 block_root
             )));
         };