Throw a new exception on server using:
Response.StatusCode = 500
Response.StatusDescription = ex.Message()
I believe that the StatusDescription is returned to the Ajax call...
Example:
Try
Dim file As String = Request.QueryString("file")
If String.IsNullOrEmpty(file) Then Throw New Exception("File does not exist")
Dim sTmpFolder As String = "Temp\" & Session.SessionID.ToString()
sTmpFolder = IO.Path.Combine(Request.PhysicalApplicationPath(), sTmpFolder)
file = IO.Path.Combine(sTmpFolder, file)
If IO.File.Exists(file) Then
IO.File.Delete(file)
End If
Catch ex As Exception
Response.StatusCode = 500
Response.StatusDescription = ex.Message()
End Try