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/text-lcov/index.js

@@ -0,0 +1,14 @@
+/*
+ Copyright 2012-2015, Yahoo Inc.
+ Copyrights licensed under the New BSD License. See the accompanying LICENSE file for terms.
+ */
+const util = require('util');
+const LcovOnly = require('../lcovonly');
+
+function TextLcov(opts) {
+    opts.file = '-';
+    LcovOnly.call(this, opts);
+}
+
+util.inherits(TextLcov, LcovOnly);
+module.exports = TextLcov;