Do not fail the session when the data channel has been closed
Do not fail the session when the data channel has been closed

Rationale for this change is that the data channel should not close
prematurely any more since we're correctly managing the underlying
buffers. It's not trivial to correctly handle multiple failure events
that have the same cause. Thus, we'll just ignore this event for now.

threema-ch/threema-web,threema-ch/threema-web,threema-ch/threema-web,threema-ch/threema-web,threema-ch/threema-web