Default streams (streams which new users are subscribed to by default) can not be made private. On web, when trying to change the permissions of a default stream to private, the following error is displayed:

Save failed: Default streams cannot be made private.

On mobile, the operation fails silently which may cause the user to believe they have changed the permissions of a stream when in fact they haven't.

When the request to set the default stream to private is sent, zulip server responds with this response:

{"result":"error","msg":"Default streams cannot be made private.","code":"BAD_REQUEST"}

I'm still getting familiar with how API errors are handled on mobile, but perhaps we should handle them here?

Steps to reproduce:

Log in to a development Zulip server as an admin and try setting a default stream (#design, #devel, #social, #support, #Verona in the default dev environment) to private in the stream's settings. After saving go back into the stream settings and note that the setting has reverted, but there is no feedback detailing why.

Note: I got caught up by this when working on #5250 today, which is relevant in that it introduces the ability to save additional stream settings. It would be helpful to figure this out before fully implementing it.