Need better error message on adding users to groups

I tried adding a user who isn't registered yet to one of my FireCloud groups and got this error message (with the name changed so I can post it in public):

Error: Sam call to HttpMethod(PUT) /api/group/MethodsDev/member/foo@broadinstitute.org failed with error HttpEntity.Strict(application/json,{"statusCode":400,"source":"sam","causes":[],"stackTrace":[],"message":"foo@broadinstitute.org not found"})

Clearly, this should be cleaned up when presented to the user....