jQuery deferred AJAX & 500 server errors

Question

Trying to figure out how to debug generic 500 errors from my .ASMX

JS:

function TestError() {
    return $.ajax({
        type: "POST",
        url: 'Order.asmx/TestError',
        contentType: "application/json; charset=utf-8"
    });
}

$.when(TestError()).then(function (err) {
    console.log(err);
});

C#

[WebMethod]
[ScriptMethod(ResponseFormat = ResponseFormat.Json)]
public object TestError()
{
    try
    {
        throw new Exception("Testing error");
    }
    catch (Exception ex)
    {
        return ex;
    }
}

Result:

It's not even logging the err argument. Any ideas?

Edit

adding the error to the original Ajax call is getting me logging info, but still generic & useless.

http://i42.tinypic.com/2v27fgo.jpg

Edit 2

Answered my own question and have asked a follow-up:

Removing properties of an inherited object in C#

Answer 1

After a kick in the right direction and further digging I actually found this problem was twofold.

First, Dave Ward actually pointed me in the right direction in suggesting I turn off customErrors in the web.config. It was actually not that, but very close.

The actual culprit is the Elmah error handling module. After disabling it I was able to relay custom errors regardless of whether customErrors was on or off.

The second issue was that although I could now handle the errors, I still couldn't pass System.Exception objects. I discovered this is because the Exception.TargetSite property is not serializable.

I got around the problem by creating my own JsonException class and only including the serializable properties of the System.Exception class (abbreviated here for simplicity).

public class JsonException
{
    public string Source { get; set; }
    public string Message { get; set; }
    public string StackTrace { get; set; }

    public JsonException(Exception ex)
    {
        this.Source = ex.Source;
        this.Message = ex.Message;
        this.StackTrace = ex.StackTrace;
    }
}

[WebMethod]
[ScriptMethod(ResponseFormat = ResponseFormat.Json)]
public object TestError()
{
    try
    {
        throw new Exception("Testing error");
    }
    catch (Exception ex)
    {
        return new JsonException(ex);
    }
}

And now I finally get the data I want: