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/union.js

@@ -0,0 +1,26 @@
+var baseFlatten = require('./_baseFlatten'),
+    baseRest = require('./_baseRest'),
+    baseUniq = require('./_baseUniq'),
+    isArrayLikeObject = require('./isArrayLikeObject');
+
+/**
+ * Creates an array of unique values, in order, from all given arrays using
+ * [`SameValueZero`](http://ecma-international.org/ecma-262/7.0/#sec-samevaluezero)
+ * for equality comparisons.
+ *
+ * @static
+ * @memberOf _
+ * @since 0.1.0
+ * @category Array
+ * @param {...Array} [arrays] The arrays to inspect.
+ * @returns {Array} Returns the new array of combined values.
+ * @example
+ *
+ * _.union([2], [1, 2]);
+ * // => [2, 1]
+ */
+var union = baseRest(function(arrays) {
+  return baseUniq(baseFlatten(arrays, 1, isArrayLikeObject, true));
+});
+
+module.exports = union;