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/toIterator.js

@@ -0,0 +1,23 @@
+/**
+ * Enables the wrapper to be iterable.
+ *
+ * @name Symbol.iterator
+ * @memberOf _
+ * @since 4.0.0
+ * @category Seq
+ * @returns {Object} Returns the wrapper object.
+ * @example
+ *
+ * var wrapped = _([1, 2]);
+ *
+ * wrapped[Symbol.iterator]() === wrapped;
+ * // => true
+ *
+ * Array.from(wrapped);
+ * // => [1, 2]
+ */
+function wrapperToIterator() {
+  return this;
+}
+
+module.exports = wrapperToIterator;