Iconsets do not contain an icon for the state error_ack

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Describe the bug
In the config file nagvis/etc/nagvis.ini.php the priority to object states are defined. This includes a state called 'error_ack'. However, the iconsets do not contain a corresponding icon file for this state.

NagVis Version
<=1.9.40

Expected behavior
For each object state listed in nagvis/etc/nagvis.ini.php a corresponding icon file should be present in the iconsets.