scstuff.set-magtype.py needs to wait for acknowlegement by scevent

[Joachim Saul]

scstuff.set-magtype.py sends a journal entry requesting a change of the magnitude type. Currently the request is sent and the program exits immediately. To be on the safe side, e.g. for use within shell scripts, scstuff.set-magtype.py should wait for an acknowlegement by scevent and then either exits normally or with an error code in case of an error.