Emit error when attempting to delete in use refset
Emit error when attempting to delete in use refset

Issue #1226

dcolligan/server,david4096/ga4gh-server,david4096/ga4gh-server,david4096/ga4gh-server,ohsu-computational-biology/server,saupchurch/server,ohsu-computational-biology/server,saupchurch/server,dcolligan/server,dcolligan/server,ga4gh/server,saupchurch/server,ga4gh/server,ga4gh/server,ohsu-computational-biology/server