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/es2015.js

@@ -0,0 +1,9 @@
+'use strict';
+
+var ES = require('../').ES2015;
+
+var ops = require('../operations/2015');
+
+var expectedMissing = ['Construct', 'SetIntegrityLevel', 'TestIntegrityLevel', 'CreateArrayFromList', 'CreateListFromArrayLike', 'OrdinaryHasInstance', 'CreateListIterator', 'RegExpBuiltinExec', 'IsPromise', 'NormalCompletion'];
+
+require('./tests').es2015(ES, ops, expectedMissing);