web 请求中的“try-catch”问题 vb.net

Question

I'm making a web request that completes successfully most of the time (target$ is a URL). But occasionally my code throws a valid exception, 404 not found, if the URL target$ doesn't exist, and execution stops. The code:

Sub scrape(target$)
    Dim request As WebRequest = WebRequest.Create(target$)
    Dim response As WebResponse = request.GetResponse()
    Dim dataStream As Stream = response.GetResponseStream()
    ' Open the stream using a StreamReader for easy access.
    Dim reader As New StreamReader(dataStream)
    ' Read the content.
    Dim responseFromServer As String = reader.ReadToEnd()
    txtResponse.Text = ""
    txtResponse.Text = responseFromServer
    ' Clean up the streams and the response.
    reader.Close()
    response.Close()

end sub

The exception, if thrown, happens in the second line, "Dim response...". So I tried adding a "try-catch" as shown.

Sub scrape(target$)
    Dim request As WebRequest = WebRequest.Create(target$)
    Dim response As WebResponse = request.GetResponse()
    Try
        Dim dataStream As Stream = response.GetResponseStream()
    Catch
        exflag = True
    End Try
    ' Open the stream using a StreamReader for easy access.
    Dim reader As New StreamReader(dataStream)
    ' Read the content.
    Dim responseFromServer As String = reader.ReadToEnd()
    txtResponse.Text = ""
    txtResponse.Text = responseFromServer
    ' Clean up the streams and the response.
    reader.Close()
    response.Close()

end sub

But now when I try to compile the code, VisualStudio tells me that "datastream is not declared" and the compile fails.

What am I doing wrong and how do I catch the exception when it's thrown?

Thanks...