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/_baseFindKey.js

@@ -0,0 +1,23 @@
+/**
+ * The base implementation of methods like `_.findKey` and `_.findLastKey`,
+ * without support for iteratee shorthands, which iterates over `collection`
+ * using `eachFunc`.
+ *
+ * @private
+ * @param {Array|Object} collection The collection to inspect.
+ * @param {Function} predicate The function invoked per iteration.
+ * @param {Function} eachFunc The function to iterate over `collection`.
+ * @returns {*} Returns the found element or its key, else `undefined`.
+ */
+function baseFindKey(collection, predicate, eachFunc) {
+  var result;
+  eachFunc(collection, function(value, key, collection) {
+    if (predicate(value, key, collection)) {
+      result = key;
+      return false;
+    }
+  });
+  return result;
+}
+
+module.exports = baseFindKey;