Is it possible to mark test as failed when jscript exception occurs?

If your project properties is configured to "Stop current item" on "On Error",

function testit()
{
    try {
    	// Cause an exception to occur
        throw new Error("An exception has occurred");
    } catch (e) {
    	// Perform cleanup
        Log.Message("Catch Perform Cleanup");

    	// Log error message
    	Log.Error(e.message);
        
        Log.Message("This should not be called");
    } finally {
    	// Perform cleanup
        Log.Message("Finally Perform Cleanup");
    }

    Log.Message("This should not be called as well");
}

Then both messages e.g. "This should not be called..." will not be shown, as TestComplete will stop at Log.Error()