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/object.getownpropertydescriptors/test/index.js

@@ -0,0 +1,17 @@
+'use strict';
+
+var getDescriptors = require('../');
+var test = require('tape');
+var runTests = require('./tests');
+
+test('as a function', function (t) {
+	t.test('bad object/this value', function (st) {
+		st.throws(function () { return getDescriptors(undefined); }, TypeError, 'undefined is not an object');
+		st.throws(function () { return getDescriptors(null); }, TypeError, 'null is not an object');
+		st.end();
+	});
+
+	runTests(getDescriptors, t);
+
+	t.end();
+});