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-report/lib/watermarks.js

@@ -0,0 +1,14 @@
+/*
+ Copyright 2012-2015, Yahoo Inc.
+ Copyrights licensed under the New BSD License. See the accompanying LICENSE file for terms.
+ */
+module.exports = {
+    getDefault() {
+        return {
+            statements: [50, 80],
+            functions: [50, 80],
+            branches: [50, 80],
+            lines: [50, 80]
+        };
+    }
+};