HTTP push not implmented but fails silently

When using http as the backend and running git-fat push, it would fail
silently instead of alerting the user that it is not implemented.

This change alerts the user when the specified backend hasn't
implemented the push command.
Merge pull request #19 from abraithwaite/fix-http-push-silentfail

Fix http push silentfail
Release 0.3.2