bookmarks: Only save undo.bookmarks if bookmarks exist

Otherwise the command will abort when there is no .hg/bookmarks file.

hgext/bookmarks.py

@@ -52,6 +52,7 b' def write(repo, refs):'
     We also store a backup of the previous state in undo.bookmarks that
     can be copied back on rollback.
     '''
+    if os.path.exists(repo.join('bookmarks')):
     util.copyfile(repo.join('bookmarks'), repo.join('undo.bookmarks'))
     file = repo.opener('bookmarks', 'w+')
     for refspec, node in refs.items():