REST API for Add user to group returns error 400 instead of 404 when the user does not exist

h3. Issue Summary REST API for Add user to group returns error 400 instead of 404 when the user does not exist. According to the documentation of JIRA 8.5.3|https://docs.atlassian.com/software/jira/docs/api/REST/8.5.3/api/2/group-addUserToGroup when the user or group does not exist, an error 404...