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/es2016.js

@@ -0,0 +1,16 @@
+'use strict';
+
+var ES2015 = require('./es2015');
+var assign = require('./helpers/assign');
+
+var ES2016 = assign(assign({}, ES2015), {
+	// https://github.com/tc39/ecma262/pull/60
+	SameValueNonNumber: function SameValueNonNumber(x, y) {
+		if (typeof x === 'number' || typeof x !== typeof y) {
+			throw new TypeError('SameValueNonNumber requires two non-number values of the same type.');
+		}
+		return this.SameValue(x, y);
+	}
+});
+
+module.exports = ES2016;