Task is unusable due status error caused by a recoverable error

Errors on using termtagger / spellchecker on import cause the task to be set to status error.

Instead such errors should not set the task to status error, since they are recoverable because the workers are running in a loop until a segments are processed.

Solution: in the worker looping over the segments exceptions should catched and logged as warning but not lead to a exception stop of the worker.

Example which caused the problem and can be faked for reproduction: