D5690: py3: use regular expression to deal with ENOENT formatting change

[Yuya Nishihara]

>    $ hg log -b --cwd=inexistent default
> -  abort: $ENOENT$: 'inexistent'
> +  abort: \$ENOENT\$: ('inexistent'|inexistent) (re)

That's our fault. The exception is caught as IOError on Python 3, and our
formatting of IOError and OSError are slightly different!

Maybe we should unify them.