title: How to show a error when a modal popup extender is active
body: <p>I have a <code>ModalPopupExtender</code> to create new users. When the user enters the new user details and tries to enter the same email Id which is already registered then I want to show a error.</p>
  <p>Create user:</p>
  <pre><code>  if (emailcount != 0)
              {
                  
                  Page.ClientScript.RegisterStartupScript(GetType(), &quot;UserDialogScript&quot;, 
  
  &quot;alert(\&quot;User already exists!\&quot;);&quot;, true);
              }
  </code></pre>
  <p>I check whether the email is already used, if it is used I want to pop up the error but the page stays still with the ModalPopupExtender on the top. I am not able to do anything after that. How can I display the error?</p>