You could try slightly different approach. Instead of checking whether the response has succeeded or not you could place one HiddenField control on your page and assign its value with some default error message, the on every successful postback you should replace the text with something different. Then you could check on the client what is the value of this HiddenField and based on this value you will change the visibility of the panel.
For example on successful postback you could set the value to "Normal" or "Expected" and if the user exists or some DB error is thrown you could set the value to "Abnormal" or "UnExpected". Then you could check if the value is "UnExpected" for example you could hide the panel with similar to this code:
panel.style.display = 'block;
panel.style.display = 'none;
Give this approach a try and check whether this is the desired behavior.
All the best,
the Telerik team
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