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/has-symbols/test/index.js

@@ -0,0 +1,22 @@
+'use strict';
+
+var test = require('tape');
+var hasSymbols = require('../');
+var runSymbolTests = require('./tests');
+
+test('interface', function (t) {
+ 	t.equal(typeof hasSymbols, 'function', 'is a function');
+	t.equal(typeof hasSymbols(), 'boolean', 'returns a boolean');
+	t.end();
+});
+
+test('Symbols are supported', { skip: !hasSymbols() }, function (t) {
+	runSymbolTests(t);
+	t.end();
+});
+
+test('Symbols are not supported', { skip: hasSymbols() }, function (t) {
+	t.equal(typeof Symbol, 'undefined', 'global Symbol is undefined');
+	t.equal(typeof Object.getOwnPropertySymbols, 'undefined', 'Object.getOwnPropertySymbols does not exist');
+	t.end();
+});