Log: controller: improve messages when metadata cache update fails

Previously, metadata_cache_update() or ra_param_from_xml() would log an error,
then controld_get_rsc_metadata() (but not the other caller,
process_lrm_event()) would log another warning with the agent info.

Combine these into a single message always logged by metadata_cache_update(),
which also has been renamed to controld_cache_metadata().