openxchange: display full error message
Summary:
The previous code splits the error text on the first slash, but newer
xchange servers could return error messages with slashes in the text.
CCBUG: 333408
Reviewers: KDE PIM, mlaurent
Reviewed By: KDE PIM, mlaurent
Subscribers: mlaurent
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D4381