Handle invalid block numbers in eth_getTransactionCount

lib/blockchain_double.js

@@ -237,6 +237,8 @@ BlockchainDouble.prototype.getBlock = function(number, callback) {
       return this.latestBlock(callback);
     } else if (number === "earliest") {
       return this.data.blocks.first(callback);
+    } else {
+      process.nextTick(callback, new Error("Invalid `blockNumber`: \"" + number + "\""));
     }
   }
 };