Better description for assert in frame-session genesis (#8399)

* Better description for assert in frame-session genesis

There is an assert that checks that an account exists, after setting a
key. However, this assert isn't very self-descriptive.

* Update frame/session/src/lib.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

substrate/frame/session/src/lib.rs

@@ -442,7 +442,11 @@ decl_storage! {
 			for (account, val, keys) in config.keys.iter().cloned() {
 				<Module<T>>::inner_set_keys(&val, keys)
 					.expect("genesis config must not contain duplicates; qed");
-				assert!(frame_system::Pallet::<T>::inc_consumers(&account).is_ok());
+				assert!(
+					frame_system::Pallet::<T>::inc_consumers(&account).is_ok(),
+					"Account ({:?}) does not exist at genesis to set key. Account not endowed?",
+					account,
+				);
 			}
 
 			let initial_validators_0 = T::SessionManager::new_session(0)