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