When doing AJAX through Dojo we can pass two callbacks, one to execute after a successfull request and one to execute after an error:
dojo.xhr("GET",{
url: myURL,
content: messageContents,
load: function(returnData, ioArgs){
//This is called on success
},
error: function(returnData, ioArgs){
//This is called on failure
}
});
I couldn't find in the documentation what is defined as an error. I'd guess anything with a return code >= 400 but I'm not sure.
Generally speaking, an unsuccessful HTTP response code. The determination is made by calling dojo._isDocumentOk which as you'll see basically accepts 2xx and 304 plus some browser-quirk stuff.