This works around this error:

    snapshot file missing, is maim installed?
    Traceback (most recent call last):
      File "/home/anarcat/bin/publish", line 847, in <module>
        main(args)
      File "/home/anarcat/bin/publish", line 810, in main
        history_append(args.token, uri or None)
    UnboundLocalError: local variable 'uri' referenced before assignment

... which occurs when a screenshot is canceled, and because `uri`
never gets set because args.files is empty.

The real fix is to abort earlier if args.files is empty, but we should
nonetheless save the right token to history.

This happens when the screenshot tool aborts or fails.

It is not exclusive with the other commands!

The + sign doesn't actually work for normal URLs: it only works for
query strings, which is not the case here.