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/sortedUniq.js

@@ -0,0 +1,24 @@
+var baseSortedUniq = require('./_baseSortedUniq');
+
+/**
+ * This method is like `_.uniq` except that it's designed and optimized
+ * for sorted arrays.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.0.0
+ * @category Array
+ * @param {Array} array The array to inspect.
+ * @returns {Array} Returns the new duplicate free array.
+ * @example
+ *
+ * _.sortedUniq([1, 1, 2]);
+ * // => [1, 2]
+ */
+function sortedUniq(array) {
+  return (array && array.length)
+    ? baseSortedUniq(array)
+    : [];
+}
+
+module.exports = sortedUniq;