Maildispatcher: escape '<' and '>' in error messages.

Summary:
The plasma notification popup for the (new) error message

"The server said: 4.1.1 <doesnotexit@example.com>: Recipient address
 rejected: User unknown in local recipient table"

would truncate after 4.1.1. for lack of escaping.

Test Plan: Sending a mail to a non existing user on a kolab server.

Reviewers: dvratil, lbeltrame, mlaurent, fvogt, vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D15529