[INTERNAL] ensure that a connection is not reestablished on failure

As the current network layer will create a new connection we have
to prevent this behavior one layer above in the ActivitySequencer.

This patch ensures that no activities will be send after a connection
failure unless the user is registered again.

TODO: shutdown the session / kick the user


Change-Id: I27e52e63d141552f6d90812eb26fbe5b0f1b7df9
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/931
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>