lock: fix etcd error handling

The type conversion with the new locksmith/etcd.Error introduced in
a621605a94055a2e8ef3ebcdde85b277fd564adf unfortunately does not work,
since the underlying type is still from the go-etcd library in this
case. Ideally go-etcd should not be exposed anywhere except
locksmith/etcd so I went down a path of wrapping every go-etcd call and
converting go-etcd.EtcdError to etcd.Error, but this turned into a
rabbit hole (since it implies that go-etcd.Response should also be
converted, which necessitates locksmith/etcd.Response, ...)

For now, revert to just leaking the go-etcd types into the lock package.
To be revisited....
Merge pull request #43 from jonboulle/fix_etcd_error

lock: fix etcd error handling