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/_parent.js

@@ -0,0 +1,16 @@
+var baseGet = require('./_baseGet'),
+    baseSlice = require('./_baseSlice');
+
+/**
+ * Gets the parent value at `path` of `object`.
+ *
+ * @private
+ * @param {Object} object The object to query.
+ * @param {Array} path The path to get the parent value of.
+ * @returns {*} Returns the parent value.
+ */
+function parent(object, path) {
+  return path.length < 2 ? object : baseGet(object, baseSlice(path, 0, -1));
+}
+
+module.exports = parent;