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