tls: throw an error, not string

Reviewed-By: Fedor Indutny <fedor@indutny.com>

lib/_tls_wrap.js

@@ -738,7 +738,7 @@ Server.prototype.setOptions = function(options) {
 // SNI Contexts High-Level API
 Server.prototype.addContext = function(servername, context) {
   if (!servername) {
-    throw 'Servername is required parameter for Server.addContext';
+    throw new Error('Servername is required parameter for Server.addContext');
   }
 
   var re = new RegExp('^' +