All of the desired modifications could be done in the localization files of the Editor. The content of the error message could be found inside "RadEditor.Main.resx" and the text of the buttons - inside "RadEditor.Dialogs.resx". More information regarding the localization and how to modify it is available here:
Using Global Resource Files
Another option is to make the changes in the code behind, e.g:
sender, EventArgs e)
"Your custom message here."
I hope this helps.
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