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/isSet.js

@@ -0,0 +1,27 @@
+var baseIsSet = require('./_baseIsSet'),
+    baseUnary = require('./_baseUnary'),
+    nodeUtil = require('./_nodeUtil');
+
+/* Node.js helper references. */
+var nodeIsSet = nodeUtil && nodeUtil.isSet;
+
+/**
+ * Checks if `value` is classified as a `Set` object.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.3.0
+ * @category Lang
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `value` is a set, else `false`.
+ * @example
+ *
+ * _.isSet(new Set);
+ * // => true
+ *
+ * _.isSet(new WeakSet);
+ * // => false
+ */
+var isSet = nodeIsSet ? baseUnary(nodeIsSet) : baseIsSet;
+
+module.exports = isSet;