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/_listCacheClear.js

@@ -0,0 +1,13 @@
+/**
+ * Removes all key-value entries from the list cache.
+ *
+ * @private
+ * @name clear
+ * @memberOf ListCache
+ */
+function listCacheClear() {
+  this.__data__ = [];
+  this.size = 0;
+}
+
+module.exports = listCacheClear;