Fails silently when creating/editing relationships with non-Person names

1. Steps to produce

1. Login as admin
2. Click to register a patient
3. Select a relationship type
4. Fill relationship 'Person name' with a name that does not match a person (eg. "Abcdefg")
5. Save the patient

And observe that the patient was registered with no relationships, thereby failing silently.

In summary: It is possible to 1) submit as the relationship name a string that isn’t a pointer to any person in OpenMRS and it will fail silently (saving nothing) or worse 2) save a relationship with an unintended person. (duplicated by mksd & mogoodrich's teams in RefApp QA environment).

Because this could cause an unintended relationship link with an unrelated person, this is a safety concern, so it is a blocker.