Tweak some assertions to better match their error messages (#2822)

* Change assertion condition to match message

Refactor the condition so that the expression is easier to compare to
the panic message.

* Change comparison order to match panic message

Change the order of the equality so that it matches the panic message.

zebra-chain/src/history_tree.rs

@@ -200,7 +200,7 @@ impl NonEmptyHistoryTree {
             .expect("block must have coinbase height during contextual verification");
 
         assert!(
-            height - self.current_height == 1,
+            Some(height) == self.current_height + 1,
             "added block with height {:?} but it must be {:?}+1",
             height,
             self.current_height

zebra-chain/src/primitives/zcash_history.rs

@@ -184,7 +184,7 @@ impl<V: Version> Tree<V> {
         let network_upgrade = NetworkUpgrade::current(self.network, height);
 
         assert!(
-            self.network_upgrade == network_upgrade,
+            network_upgrade == self.network_upgrade,
             "added block from network upgrade {:?} but history tree is restricted to {:?}",
             network_upgrade,
             self.network_upgrade