Fixed OSError "No such file or directory: None" and make IOError consistent. The None error occurs when the current working directory is no longer available, e.g. because it was removed in another shell.

#!/bin/sh

hg init a
echo a > a/a
hg --cwd a ci -A -m a
hg clone a b
echo b > b/b
hg --cwd b ci -A -m b
chmod 100 a/.hg
hg --cwd b push ../a
chmod 700 a/.hg