cannot override an existing dictionary in upgrade

src/rougail/update.py

@@ -76,6 +76,8 @@ class RougailUpgrade:
         for filename in filenames:
             xmlsrc = join(srcfolder, filename)
             xmldst = join(dstfolder, filename)
+            if isfile(xmldst):
+                raise Exception(f'cannot update "{xmlsrc}" destination file "{xmldst}" already exists')
             try:
                 parser = XMLParser(remove_blank_text=True)
                 document = parse(xmlsrc, parser)