always return an error when there's a message

As reported in code review, returning an error is the best indicator that
something has gone wrong.

BACK-2500

api/clinicianInvites.go

@@ -394,7 +394,9 @@ func (a *Api) sendClinicianConfirmation(req *http.Request, confirmation *models.
 	}
 
 	if !a.createAndSendNotification(req, confirmation, emailContent) {
-		return STATUS_ERR_SENDING_EMAIL, nil
+		// TODO: better to re-work createAndSendNotification to return a
+		// proper error.
+		return STATUS_ERR_SENDING_EMAIL, fmt.Errorf("sending email")
 	}
 
 	a.logMetric("clinician_invite_sent", req)