lxd/migrate/instance: Improve error handling and logging in migrationSink.Do
lxd/migrate/instance: Improve error handling and logging in migrationSink.Do

 - Switches to contextual logging.
 - Improves error messages

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

hallyn/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd