"registration-required" raw error message is not super clear for users when attempting to join a restricted group chat room without invitation

Versions

OS: Fedora 42
Gajim: 2.3.3 from Flathub
GTK Version: 4.18.6
Adw Version: 1.7.5
GLib Version: 2.84.3
Pango Version: 1.56.4
PyGObject Version: 3.52.3
cairo Version: 1.18.4
pycairo Version: 1.28.0
python-nbxmpp Version: 6.2.0
libsoup Version: 3.6.5

Steps to reproduce the problem

Today I tried to manually join a chatroom that someone tried inviting me to (and they didn't know they had insufficient privileges to actually invite me, due to #11605), by using the XMPP URL (as seen in #12391 (closed)), and upon clicking the "Start New Chat" button, I got the error below, which seems to be a raw error rather than something worded/presented to be understandable by anyone.

Expected behavior

Use a custom, well-written (and translatable) error message string, something roughly like this maybe? :

You are not registered as an allowed member of this room.
A moderator or administrator from that room needs to send an invitation for your user account to join this discussion.

(There are many ways to phrase this, and maybe the notion of moderator vs admin vs owner I suppose, but you get the idea).

Actual behavior

This unformatted, raw "registration-required" string is not really user-friendly:

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