Instead of manually logging errors after each potentially
error-producing function, use the error-callback. The error texts
are accumulated in the QMLManager object for further use.
The text is transported to the QMLManager object via a queued
connection. Thus, errors can be reported from other threads
without having to deal with manual locking.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
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