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/lcov/index.js

@@ -0,0 +1,28 @@
+/*
+ Copyright 2012-2015, Yahoo Inc.
+ Copyrights licensed under the New BSD License. See the accompanying LICENSE file for terms.
+ */
+const LcovOnlyReport = require('../lcovonly');
+const HtmlReport = require('../html');
+
+function LcovReport() {
+    this.lcov = new LcovOnlyReport({ file: 'lcov.info' });
+    this.html = new HtmlReport({ subdir: 'lcov-report' });
+}
+
+['Start', 'End', 'Summary', 'SummaryEnd', 'Detail'].forEach(what => {
+    const meth = 'on' + what;
+    LcovReport.prototype[meth] = function(...args) {
+        const lcov = this.lcov;
+        const html = this.html;
+
+        if (lcov[meth]) {
+            lcov[meth](...args);
+        }
+        if (html[meth]) {
+            html[meth](...args);
+        }
+    };
+});
+
+module.exports = LcovReport;