slicecamd logs spurious ERRORs for RA/DECL on every no-target publish

[astronomerdave]

Sequencer::Sequence::publish_targetinfo() intentionally publishes an empty TARGETINFO message when seq state is not READY/RUNNING, but Slicecam::Interface::handletopic_targetinfo() (slicecamd/slicecam_interface.cpp:1010) requests both keys unconditionally, and Common::extract_telemetry_value logs at()'s throw as an ERROR. Behavior is already correct (coords fall through to NAN) — this is log noise only. Fix is probably a jmessage.contains() guard in slicecamd, not a change to the shared template. last run on flexure branch