Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/asynckit/lib/state.js

@@ -0,0 +1,37 @@
+// API
+module.exports = state;
+
+/**
+ * Creates initial state object
+ * for iteration over list
+ *
+ * @param   {array|object} list - list to iterate over
+ * @param   {function|null} sortMethod - function to use for keys sort,
+ *                                     or `null` to keep them as is
+ * @returns {object} - initial state object
+ */
+function state(list, sortMethod)
+{
+  var isNamedList = !Array.isArray(list)
+    , initState =
+    {
+      index    : 0,
+      keyedList: isNamedList || sortMethod ? Object.keys(list) : null,
+      jobs     : {},
+      results  : isNamedList ? {} : [],
+      size     : isNamedList ? Object.keys(list).length : list.length
+    }
+    ;
+
+  if (sortMethod)
+  {
+    // sort array keys based on it's values
+    // sort object's keys just on own merit
+    initState.keyedList.sort(isNamedList ? sortMethod : function(a, b)
+    {
+      return sortMethod(list[a], list[b]);
+    });
+  }
+
+  return initState;
+}