Add input validation to createLookup function

Author: alexandru-pocovnicu

Sprint-2/implement/lookup.js

@@ -1,4 +1,7 @@
 function createLookup(countryCurrency) {
+  if (!Array.isArray(countryCurrency)) {
+    throw new Error("Argument must be an array");
+  }
   let countryCurrencyPairs = {};
   for (const pair of countryCurrency) {
     let country = pair[0];