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/istanbul-reports/lib/none/index.js

@@ -0,0 +1,7 @@
+/*
+ Copyright 2012-2015, Yahoo Inc.
+ Copyrights licensed under the New BSD License. See the accompanying LICENSE file for terms.
+ */
+function NoneReport() {}
+
+module.exports = NoneReport;