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/uniq.js

@@ -0,0 +1,25 @@
+var baseUniq = require('./_baseUniq');
+
+/**
+ * Creates a duplicate-free version of an array, using
+ * [`SameValueZero`](http://ecma-international.org/ecma-262/7.0/#sec-samevaluezero)
+ * for equality comparisons, in which only the first occurrence of each element
+ * is kept. The order of result values is determined by the order they occur
+ * in the array.
+ *
+ * @static
+ * @memberOf _
+ * @since 0.1.0
+ * @category Array
+ * @param {Array} array The array to inspect.
+ * @returns {Array} Returns the new duplicate free array.
+ * @example
+ *
+ * _.uniq([2, 1, 2]);
+ * // => [2, 1]
+ */
+function uniq(array) {
+  return (array && array.length) ? baseUniq(array) : [];
+}
+
+module.exports = uniq;