In Mercurial, if you have no bookmarks in your working copy and run `arc bookmarks` or `arc feature`, you get an error:
```
$ arc bookmark
Exception
Command failed with error #255!
COMMAND
HGPLAIN=1 hg log -l 1 --template '{node}{date|hgdate}{p1node}{desc|firstline}{desc}' -r ''\''no bookmarks'\'''
```
Instead, we should detect that there are no bookmarks correctly.