Catch urllib's HTTPException and give a meaningful error message to the user. Usually urllib only throws HTTPError exceptions, which already are a subclass of IOError.

#!/bin/sh

mkdir t
cd t
hg init
echo a > a
hg add a
hg commit -m "test" -d "1000000 0"
hg verify
hg parents
hg status
hg rollback
hg verify
hg parents
hg status