evm: Fix minor issues from PR#4

Author: bruce-riley

evm/src/NttManager/ManagerBase.sol

@@ -354,6 +354,7 @@ abstract contract ManagerBase is
         assert(this.mode() == mode);
         assert(this.chainId() == chainId);
         assert(this.endpoint() == endpoint);
+        assert(this.executor() == executor);
     }
 
     function _checkThresholdInvariants() internal view {

evm/src/NttManager/NttManager.sol

@@ -662,6 +662,8 @@ contract NttManager is INttManager, RateLimiter, ManagerBase {
 
         uint128 gasLimit = peerData.gasLimit;
         if (gasLimit == 0) {
+            // The gas limit can only be zero when a contract has been migrated from an older version,
+            // where the gasLimit was not defined in the manager peer struct and was not set during the upgrade.
             revert InvalidGasLimitZero(args.recipientChain);
         }