convert: subversion convert abort on revision not found (issue 3205)

hgext/convert/subversion.py

@@ -439,6 +439,8 b' class svn_source(converter_source):'
             if revnum < stop:
                 stop = revnum + 1
             self._fetch_revisions(revnum, stop)
+            if rev not in self.commits:
+                raise util.Abort(_('svn: revision %s not found') % revnum)
         commit = self.commits[rev]
         # caller caches the result, so free it here to release memory
         del self.commits[rev]