tls: add assert msg options.checkServerIdentity

Author: Shigeki Ohtsu

lib/_tls_wrap.js

@@ -888,7 +888,8 @@ exports.connect = function(/* [port, host], options, cb */) {
   if (!options.keepAlive)
     options.singleUse = true;
 
-  assert(typeof options.checkServerIdentity === 'function');
+  assert(typeof options.checkServerIdentity === 'function',
+         'options.checkServerIdentity is not a function');
   assert(typeof options.minDHSize === 'number',
          'options.minDHSize is not a number: ' + options.minDHSize);
   assert(options.minDHSize > 0,