Dependency fixes, and more useful output

Just one little bug fixed here: due to recent changes, lvmsync now needs a
newer version of git-version-bump, but we didn't declare that dependency.
So, anyone who upgraded from an earlier version of lvmsync as a gem wouldn't
have upgraded git-version-bump, and all hell broke loose.

In an attempt to hunt down another bug, I've also improved the reporting of
problems issuing from the lvmsync "receiver" process. This means that if
the receiver reports any problems, they'll be displayed (prefixed by
"remote:"), rather than being lost for all eternity (or irritatingly
interspersed with local output).