How To Get An Intelligible Error Message In a jQuery AJAX Call

Use xhr.responseText

Example:

function fcjDispose() {
            $.ajax({
                type: "POST",
                url: "PrintJHA.aspx/DisposeFCJ",
                data: "{}",
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                success: function (msg) {
                    alert("success");
                },
                error: function (xhr, status, error) {
                    var err = eval("(" + xhr.responseText + ")");
                    alert(err.Message);
                }
            });
            return false;
        };

Using JQuery And LitJson With ASP.NET Passing Data To ASPX

Default.aspx

   function invokeCastVoteASPX(selectedVote, successFn, errorFn) {

        var url = "/mobile/polls/PollProcessorAsync.aspx";

        //Create list of parameters in the form:
        //{"nam1":"val1","name2":"val2"}       
        jsonData = "{'selectedVote':'" + selectedVote + "'}";

        $.ajax({
            type: "POST",
            url: url,
            contentType: "application/json; charset=utf-8",
            data: jsonData,
            dataType: "json",
            success: successFn,
            error: errorFn
        });
    }


    function CastVote() {

        // Get selected vote
        var selectedVote = GetSelectedVote();
        alert(selectedVote);

        if (selectedVote != null) {
            alert(selectedVote);
            // Cast Vote           
            invokeCastVoteASPX(selectedVote, OnSuccess, OnFailed);
        }
        // Close floating div   

    }

    function OnSuccess(result) {
        alert('CallBack Occured OnSuccess');
        // Close floating div
    }

    function OnFailed(error) {
        alert('CallBack Occured OnFailed');

    }    

    function GetSelectedVote() {
        var list = document.getElementById('<%= rblPollAnswers.ClientID %>'); //Client ID of the radiolist
        var inputs = list.getElementsByTagName("input");
        var selected;
        for (var i = 0; i < inputs.length; i++) {
            if (inputs[i].checked) {
                selected = inputs[i];
                break;
            }
        }
        if (selected) {
            return selected.value;
        }

    } 
    
</script>
<asp:Label ID="lblPollQuestion" runat="server"></asp:Label>
<asp:RadioButtonList ID="rblPollAnswers" runat="server" />
<asp:Button ID="btnVote" runat="server" Text="Vote" OnClientClick="CastVote();return false;" />

PollProcessorAsync.aspx

<script runat="server">
    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);

        string input = "";

        // Make sure we were posted to
        if (Request.RequestType == "POST")
        {
            try
            {
                // Get posted input stream and convert to a readable string
                StreamReader sr = new StreamReader(Request.InputStream);
                input = sr.ReadToEnd();
            }
            catch
            {
                input = "";
            }

        }


        RenderOutput(input);
    }

    protected void RenderOutput(string request)
    {
        int success = 0;
        string errorMessage = "";
        string errorType = "application";


        if (!String.IsNullOrEmpty(request))
        {
            try
            {
                JsonData json = JsonMapper.ToObject(request);
                if (json != null && json.Count > 0)
                {
                    string selectedVote = (!json["selectedVote"].IsString) ? "" : json["selectedVote"].ToString();


                    // Begin persiting vote to database  
                    Guid userID = Guid.NewGuid();
                    DateTime dateTimeStamp = DateTime.Now;

                    // connection string
                    SqlConnection mySqlConnection = CustomerDAL.GetPrimarySqlConnection();

                    try
                    {
                        try
                        {
                            mySqlConnection.Open();
                        }
                        catch (Exception)
                        {
                            mySqlConnection = CustomerDAL.GetSecondarySqlConnection();
                            mySqlConnection.Open();
                        }

                        // Sql Command object initialized with SQL INSERT
                        SqlCommand mySqlCommand = new SqlCommand("INSERT INTO PollUserResponses (UserID, PollAnswerID, DateTimeStamp) VALUES (@UserID, @PollAnswerID, @DateTimeStamp)", mySqlConnection);
                        mySqlCommand.Parameters.AddWithValue("@UserID", userID);
                        mySqlCommand.Parameters.AddWithValue("@PollAnswerID", Convert.ToInt16(selectedVote));
                        mySqlCommand.Parameters.AddWithValue("@DateTimeStamp", dateTimeStamp);

                        // Execute
                        mySqlCommand.ExecuteNonQuery();

                        success = 1;
                        errorType = "none";
                        errorMessage = "";

                    }
                    catch (Exception ex)
                    {
                        errorMessage = ex.Message;
                    }
                    finally
                    {
                        // close the Sql Connection
                        mySqlConnection.Close();
                    }

                }
            }
            catch (System.Collections.Generic.KeyNotFoundException)
            {
                errorMessage = "KeyNotFoundException";
            }
            catch (Exception ex)
            {
                errorMessage = ex.Message;
            }
        }


        string jsonOutputString = @"
{
	'd': {
		's':" + success.ToString() + @", 
		'e': {
			't': '" + errorType + @"',
			'msg': '" + errorMessage + @"'			
		}			
	}
}";


        JsonData jsonOutput = JsonMapper.ToObject(jsonOutputString);

        string output = jsonOutput.ToJson();

        Response.Clear();
        Response.ContentType = "text/plain";
        Response.Write(output);
        Response.End();
    }	
	
</script>

Great Resource For Consuming ASPX in ASP.NET using jQuery