ENH : make error message useful
ENH : make error message useful

NSLS-II/metadatastore,arkilic/metadatastore,ericdill/databroker,hhslepicka/metadatastore,tacaswell/metadataStore,ericdill/databroker,hhslepicka/metadatastore,tacaswell/metadataStore,NSLS-II/metadatastore,arkilic/metadatastore