Add information about errors

From our first experiences with GERBIL, we have to face the problem, that some annotators produce errors during the experiment. We think, that, additionally to store the error count, it would be very helpful for the annotators developers to report the errors.

This would include the following tasks
- store errors in database
- create the possibility to store key value pairs inside our database (which would make the extension of the database much easier)
- add the key value pairs as map or as string to the experiment task result class
- show errors of an experiment task in the result table of an experiment
- add a logic with which the row of an experiment result can be extended showing additional information (and the possibility to collapse the row after the extension)