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-lib-source-maps/lib/transform-utils.js

@@ -0,0 +1,20 @@
+/*
+ Copyright 2015, Yahoo Inc.
+ Copyrights licensed under the New BSD License. See the accompanying LICENSE file for terms.
+ */
+'use strict';
+
+function getUniqueKey(pathname) {
+    return pathname.replace(/[\\/]/g, '_');
+}
+
+function getOutput(cache) {
+    return Object.keys(cache).reduce((output, key) => {
+        const item = cache[key];
+        return Object.assign(output, {
+            [item.file]: item.mappedCoverage
+        });
+    }, {});
+}
+
+module.exports = { getUniqueKey, getOutput };