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/test/es7.js

@@ -0,0 +1,18 @@
+'use strict';
+
+var test = require('tape');
+
+var ES = require('../');
+var ES7 = ES.ES7;
+var ES2016 = ES.ES2016;
+var ES7entry = require('../es7');
+
+test('legacy es7 export', function (t) {
+	t.equal(ES7, ES2016, 'main ES7 === main ES2016');
+	t.end();
+});
+
+test('legacy es7 entry point', function (t) {
+	t.equal(ES7, ES7entry, 'main ES7 === ES7 entry point');
+	t.end();
+});