How to raise onError from chunking success endpoint in Fine-uploader - fine-uploader

Question

In my application, when the chunking-success-endpoint returns JSON like {"error": "reason..."} (for example the server failed to put the chunks back together), the onError event is not being called.
Just to clarify, the onError event is called if individual chunks fail. It's the final success request that I'm having trouble with.
I'm not sure what I'm missing. Should I be handling this some other way?
My configuration is below. Thanks in advance!
var uploader = $('#fine-uploader').fineUploader({
template: 'qq-template',
debug: true,
callbacks: {
onAllComplete: function(succeeded, failed) {
$('#doclib_tree').jstree(true).refresh()
if (failed.length > 0) {
alert("Error: Some files were not uploaded");
} else {
if (succeeded.length > 0 ) {
alert("Successfully uploaded " + succeeded.length + " file(s)");
}
this.reset();
toggle_upload();
}
},
onError: function(id, name, errorReason, xhrOrXdr) {
alert("Error uploading " + name + ". Reason: " + errorReason)
},
onSubmit: function(id, name) {
var promise = new qq.Promise();
var dest = document.getElementById('dest_label').innerHTML.replace(/ > /g, "/")
$.ajax({
'type': 'POST',
'async': false,
'url': "/documents/exists" ,
'data': {
'parents' : get_path(dest),
'name': name,
},
'success': function(data, textStatus, jqHXR) {
if ( check_session(data) ) {
promise.failure();
} else {
if (confirm("The file '" + dest + "/" + name + "' already exists. Replace?"))
promise.success();
else
promise.failure();
}
},
'error': function(jqHXR, textStatus, errorThrown) {
promise.success();
}
});
return promise;
},
},
chunking: {
enabled: true,
partSize: 20000000, // 20MB
success: {
endpoint: "documents/upload?success=1",
}
},
resume: {
enabled: true,
recordsExpireIn: 1, // 1 day
},
request: {
endpoint: "/documents/upload",
},
autoUpload: true
});

Answer 1

From the documentation under the chunking success section:
For successful responses, you may return an empty body with a status of 200-204. Any other status code is determined to be a failure.
In other words, to indicate failure, you must return a status code that indicates failure. Pick the most appropriate status code given the context of the error. At that point, Fine Uploader should determine that the file has failed in some way, and your onError handler will be called with the error property returned by your server.