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....


/cc @barakmich
:hankey: