Don't stop processing changesets if there is an error

I couldn't reproduce #1895, so i looked through the changes that were made before the release of 1.2.11 and noticed that we had just introduced the changeset queue..

There are multiple places in the code for processing incoming changesets, where an error halts the current pad's queue, because the queue callback isn't called. So, I fixed that and now ask @rchllc and @ldidry to test this branch (`git pull origin && git checkout fix/handle-message-continuation`)