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/_baseNth.js

@@ -0,0 +1,20 @@
+var isIndex = require('./_isIndex');
+
+/**
+ * The base implementation of `_.nth` which doesn't coerce arguments.
+ *
+ * @private
+ * @param {Array} array The array to query.
+ * @param {number} n The index of the element to return.
+ * @returns {*} Returns the nth element of `array`.
+ */
+function baseNth(array, n) {
+  var length = array.length;
+  if (!length) {
+    return;
+  }
+  n += n < 0 ? length : 0;
+  return isIndex(n, length) ? array[n] : undefined;
+}
+
+module.exports = baseNth;