added some robustness to dpbxbackend.py

[nils-tekampe]

My ISP disconnects my internet connection every 24h and assigns a new IP. So far, backups to dropbox that took longer than 24h failed in this moment as the socket was gone. The changes in this commit should improve the behavior of the backend in these situations. This code is currently in use in my backup system and will be tested over the next few weeks. It is possible that more fixes will be commited to this branch.

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This change is