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/lodash/_iteratorToArray.js

@@ -0,0 +1,18 @@
+/**
+ * Converts `iterator` to an array.
+ *
+ * @private
+ * @param {Object} iterator The iterator to convert.
+ * @returns {Array} Returns the converted array.
+ */
+function iteratorToArray(iterator) {
+  var data,
+      result = [];
+
+  while (!(data = iterator.next()).done) {
+    result.push(data.value);
+  }
+  return result;
+}
+
+module.exports = iteratorToArray;