code: public PersistedFace getFace(String personGroupId, UUID personId, UUID persistedFaceId) {
          return getFaceWithServiceResponseAsync(personGroupId, personId, persistedFaceId).toBlocking().single().body();
      }
docs: Retrieve information about a persisted face (specified by persistedFaceId, personId and its belonging personGroupId).
  
  @param personGroupId Id referencing a particular person group.
  @param personId Id referencing a particular person.
  @param persistedFaceId Id referencing a particular persistedFaceId of an existing face.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @throws APIErrorException thrown if the request is rejected by server
  @throws RuntimeException all other wrapped checked exceptions if the request fails to be sent
  @return the PersistedFace object if successful.