undo/redo get permanently disabled after please wait of heck

[gordonwoodhull]

The disable/enable solution in #1965 for errors on double-undo #1876 is correct in itself.

However, in the too-common case where we still run into a concurrency problem and the "please wait of heck" appears, and one clicks on the background to cancel, the keys end up disabled forever.

I'm not sure what to suggest here. I don't want to dig the hole deeper and automatically reenable those keys. (The best place to reenable them would be when the "curtain" goes way in progress.js / set_curtain().)

So I'm just opening this issue for further thought and discussion; it's not actionable yet.

[gordonwoodhull]

There is a larger concurrency problem and it might be better to set a lock on all commands that replace the session. But that's work.

[gordonwoodhull]

I'm currently seeing some other bug where undo fails the first time and then is disabled forever. Investigating.

[gordonwoodhull]

The case was where there is no previous version. The command gets disabled and never reenabled.

So I'm moving the disable into the has-previous check, and while I'm at it, I'm also trying using Promise.finally instead of Promise.then to see if it fixes this issue.

EDIT: Could not repro this issue, keeping .finally because it is more correct. Fixed the no previous/next version problem.