fix(api): raise SystemExit in manage.py script

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2467>

api/manage.py

@@ -12,4 +12,4 @@ warnings.warn(
 )
 
 if __name__ == "__main__":
-    SystemExit(main())
+    raise SystemExit(main())

changes/changelog.d/api-raise-system-exit.bugfix

@@ -0,0 +1 @@
+Raise SystemExit exception in API manage.py script