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/_baseSetData.js

@@ -0,0 +1,17 @@
+var identity = require('./identity'),
+    metaMap = require('./_metaMap');
+
+/**
+ * The base implementation of `setData` without support for hot loop shorting.
+ *
+ * @private
+ * @param {Function} func The function to associate metadata with.
+ * @param {*} data The metadata.
+ * @returns {Function} Returns `func`.
+ */
+var baseSetData = !metaMap ? identity : function(func, data) {
+  metaMap.set(func, data);
+  return func;
+};
+
+module.exports = baseSetData;