graph/education: Inconsistent status codes and error messages

[rhafer]

Creating an already existing user or school currently return a 500 HTTP Status. 409 might be better here.

Also the error messages for classes and schools differ slightly. We should align this

For schools:

{"error":{"code":"generalException","innererror":{"date":"2023-02-24T13:21:39Z","request-id":"cea2771d93a1c97200c84f44ed13b335"},"message":"nameAlreadyExists"}}

For Classes:

{"error":{"code":"generalException","innererror":{"date":"2023-02-24T13:01:38Z","request-id":"e9551976e0d1fab9072ee803be3755ab"},"message":"LDAP Result Code 68 \"Entry Already Exists\": "}}