I've also been looking for an answer to this, but then just now I came up with a solution myself.
Go to Sheet Properties and choose the Objects tab
In the list, select the chart object that is giving you a headache and click the "Properties..." button
Now you can click the button "Error Messages" on the General tab and set your message, either in the Default message, or other ones might be approporiate - for me the "All expressions disabled" seems to work with the extension object I am using
That's true... In my application it doesn't seem to be a problem though, the extension object I'm using is still working properly in web view. But of course I can't be sure that it wouldn't cause problems in some situations.