This is the code to check a valid HTTP response

HTTPResponse.js

function HTTPResponse( uri )
{
 var response_code ;
try {
 response_code = UrlFetchApp .fetch( uri ) .getResponseCode() .toString() ;
 }
catch( error ) {
 response_code = error .toString() .match( / returned code (\d\d\d)\./ )[1] ;
 }
finally {
 return response_code ;
 }
}