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/readable_serial_ordered.js

@@ -0,0 +1,29 @@
+var serialOrdered = require('../serialOrdered.js');
+
+// API
+module.exports = ReadableSerialOrdered;
+// expose sort helpers
+module.exports.ascending  = serialOrdered.ascending;
+module.exports.descending = serialOrdered.descending;
+
+/**
+ * Streaming wrapper to `asynckit.serialOrdered`
+ *
+ * @param   {array|object} list - array or object (named list) to iterate over
+ * @param   {function} iterator - iterator to run
+ * @param   {function} sortMethod - custom sort function
+ * @param   {function} callback - invoked when all elements processed
+ * @returns {stream.Readable#}
+ */
+function ReadableSerialOrdered(list, iterator, sortMethod, callback)
+{
+  if (!(this instanceof ReadableSerialOrdered))
+  {
+    return new ReadableSerialOrdered(list, iterator, sortMethod, callback);
+  }
+
+  // turn on object mode
+  ReadableSerialOrdered.super_.call(this, {objectMode: true});
+
+  this._start(serialOrdered, list, iterator, sortMethod, callback);
+}