Task to migrate old slidedatas to new format

Previously we stored the slidedata serialization as a hash, now we use HashWithInddiferentAccess to better deal with incoming post parameters. The old hash-serializations will lead to two sets of diverging keys and unspecified results. This task will convert all such slidedatas to the new format.
Updated the tasklist
Additional work on error handling

We handle expected and unexpected disconnects differently, we also implement the error addition in a model function.