outboxActions: Show dialog if message send fails with informative ApiError

[chrisbobbe]

outboxActions: Show dialog if message send fails with informative ApiError

This is our minimal support for #5870 in this legacy codebase.
Supporting it properly with a client-side check of the setting is
more effort than we can spare right now.

Probably more error handling is called for in general (like for
network or server issues), but #3881 ("Sending outbox messages is
fraught with issues") is complicated and it's probably best to leave
it be.

Fixes: #5870

[chrisbobbe]

Screenshots from a dev server:

a	b	c

[alya]

Can we get a more accurate error in the non-authorized case? Like the webapp front end error:

Maybe this is something to fix on the server side?

[chrisbobbe]

Thanks for catching this; #api design thread: https://chat.zulip.org/#narrow/stream/378-api-design/topic/new.20DM.20settings.3A.20a.20server.20error.20message.20seems.20wrong/near/1882954

[gnprice]

Thanks! LGTM; merging.

Definitely would be good to fix that error message; we'll pursue doing so via that #api design thread.

I made one tweak to the commit message, elaborating on why we're not doing the full-featured version:

     This is our minimal support for #5870 in this legacy codebase.
     Supporting it properly with a client-side check of the setting is
-    more effort than we can spare right now.
+    more effort than we can spare here, because it requires implementing
+    the group-based permissions system.

[chrisbobbe]

Makes sense; thanks!