Exceptional handling in ASP.NET file handler - asp.net

Question

I am processing PDF/JPG/GIF files using HTTP Handler.
A popup receives a btnId in query string and detects the file extension then calls the handler with btnId and processes the request.
Files are physical stored on server with file names in DB.
When there is an issue with the file like file not found or some error I want an alert to be displayed and opened window to be closed which is not working right now.
I know the reason and that is I am writing the alert into response which will eventually land in either object tag or Image tag.
Can some one please suggest how can this be achieved. Help will be appreciated. Thanks!!!
Public Class ViewHelpContent
Inherits System.Web.UI.Page
'Method that dynamically decides based on file extension which file viewer to render
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
Dim embed As String = String.Empty
Dim count As Integer
Dim fileName As String = String.Empty
Dim extension As String = String.Empty
Try
If Not IsPostBack Then
If Not Request.QueryString("btnId") Is Nothing Then
Dim btnId As String = Request.QueryString("btnId").ToString()
count = GetFileAttachedToButton(btnId, fileName)
extension = Path.GetExtension(fileName)
If extension.ToLower() = ".pdf" Then
embed = "<object id=""objPDFViewer"" data=""{0}{1}"" type=""application/pdf"">"
embed &= "If you are unable to view file, you can download from here"
embed &= " or there is no file available to be viewed."
embed &= "</object>"
ElseIf extension.ToLower() = ".jpeg" OrElse extension.ToLower() = ".jpg" OrElse extension.ToLower() = ".gif" Then
embed = "<img id=""objImgViewer"" src=""{0}{1}"" alt=""No file is available to be viewed."" />"
Else
ClientScript.RegisterStartupScript(Me.GetType(), "alert", "alert('No help content uploaded for this button.');", True)
Return
End If
helpContent.Text = String.Format(embed, ResolveUrl("~/ProcessHelpContent.ashx?btnId="), btnId)
End If
End If
Catch ex As Exception
ExceptionHandler.LogError(ex)
JSHelper.ShowSystemError(Me)
End Try
End Sub
End Class
Public Class ProcessHelpContent
Implements System.Web.IHttpHandler
'Sub that processes the help request and generates the requested content based on button Id
Sub ProcessRequest(ByVal context As HttpContext) Implements IHttpHandler.ProcessRequest
Dim btnId As String = String.Empty
Dim fileName As String = String.Empty
Dim contentType As String = String.Empty
Dim outputDir As String = String.Empty
Dim count As Integer
Dim strPath As FileInfo = Nothing
Dim extension As String = String.Empty
Try
If Not context.Request.QueryString("btnId") Is Nothing Then
btnId = context.Request.QueryString("btnId").ToString()
outputDir = ConfigurationManager.AppSettings("HelpContentFilesFolder")
count = GetFileAttachedToButton(btnId, fileName)
strPath = New FileInfo(outputDir & fileName)
If Not String.IsNullOrWhiteSpace(fileName) Then
If File.Exists(strPath.FullName) Then
extension = Path.GetExtension(strPath.FullName)
If context.Request.QueryString("download") = "1" Then
context.Response.AppendHeader("Content-Disposition", Convert.ToString("attachment; filename=") & fileName)
End If
context.Response.Cache.SetCacheability(HttpCacheability.NoCache)
If extension.ToLower() = ".pdf" Then
context.Response.ContentType = "application/pdf"
ElseIf extension.ToLower() = ".jpg" OrElse extension.ToLower() = ".jpeg" Then
context.Response.ContentType = "image/jpeg"
ElseIf extension.ToLower() = ".gif" Then
context.Response.ContentType = "image/gif"
End If
context.Response.TransmitFile(strPath.FullName)
context.Response.Flush()
Else
context.Response.Write("<script>alert('Uploaded help file is missing. Please re-upload or contact administrator.'); window.close();</script>")
End If
Else
context.Response.Write("<script>alert('Uploaded help file is missing. Please re-upload or contact administrator.'); window.close();</script>")
End If
End If
Catch ex As Exception
ExceptionHandler.LogError(ex)
context.Response.Write("<script>alert('Uploaded help file is missing. Please re-upload or contact administrator.'); window.close();</script>")
End Try
End Sub
ReadOnly Property IsReusable() As Boolean Implements IHttpHandler.IsReusable
Get
Return False
End Get
End Property
End Class