IMAP: Collection removal should be replayed on error.

Summary:
The review of cancelTask vs deferTask found this oddity, the other tasks
seems to use the right value.

Test Plan: none

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D12421