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/_getValue.js

@@ -0,0 +1,13 @@
+/**
+ * Gets the value at `key` of `object`.
+ *
+ * @private
+ * @param {Object} [object] The object to query.
+ * @param {string} key The key of the property to get.
+ * @returns {*} Returns the property value.
+ */
+function getValue(object, key) {
+  return object == null ? undefined : object[key];
+}
+
+module.exports = getValue;