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/es-abstract/helpers/isFinite.js

@@ -0,0 +1,3 @@
+var $isNaN = Number.isNaN || function (a) { return a !== a; };
+
+module.exports = Number.isFinite || function (x) { return typeof x === 'number' && !$isNaN(x) && x !== Infinity && x !== -Infinity; };