Properly handle defer cancelling.

- Fix issues related to "Cancel login does not work".
- Move srpauth errback to mainwindow.
- Add signal for provider setup cancel.
- Add support to cancel the soledad defer.

[Closes #4869]
[Closes #4973]
Handle closed db exception during sync.

[Closes #5130]
Merge remote-tracking branch 'refs/remotes/ivan/bug/handle-closed-soledad' into develop
Merge remote-tracking branch 'refs/remotes/ivan/bug/cancel-login-does-not-work' into develop