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/_getData.js

@@ -0,0 +1,15 @@
+var metaMap = require('./_metaMap'),
+    noop = require('./noop');
+
+/**
+ * Gets metadata for `func`.
+ *
+ * @private
+ * @param {Function} func The function to query.
+ * @returns {*} Returns the metadata for `func`.
+ */
+var getData = !metaMap ? noop : function(func) {
+  return metaMap.get(func);
+};
+
+module.exports = getData;