Better error when account index doesn't exist

lib/statemanager.js

@@ -109,7 +109,12 @@ StateManager.prototype.initialize = function(options, callback) {
   this.unlocked_accounts = (options.unlocked_accounts || []).reduce(function(obj, address) {
     // If it doesn't have a hex prefix, must be a number (either a string or number type).
     if ((address + "").indexOf("0x") != 0) {
-      address = accounts[parseInt(address)].address.toLowerCase();
+      let idx = parseInt(address)
+      let account = accounts[idx]
+      if (!account) {
+        throw new Error(`Account at index ${idx} not found. Max index available is ${accounts.length - 1}.`)
+      }
+      address = account.address.toLowerCase();
     }
 
     obj[address.toLowerCase()] = true; // can be any value