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/flatten.js

@@ -0,0 +1,22 @@
+var baseFlatten = require('./_baseFlatten');
+
+/**
+ * Flattens `array` a single level deep.
+ *
+ * @static
+ * @memberOf _
+ * @since 0.1.0
+ * @category Array
+ * @param {Array} array The array to flatten.
+ * @returns {Array} Returns the new flattened array.
+ * @example
+ *
+ * _.flatten([1, [2, [3, [4]], 5]]);
+ * // => [1, 2, [3, [4]], 5]
+ */
+function flatten(array) {
+  var length = array == null ? 0 : array.length;
+  return length ? baseFlatten(array, 1) : [];
+}
+
+module.exports = flatten;