Show error when trying to change subject (or other) without necessary privileges

Bug description

If the client does not know if the user is allowed to perform an action (changing the room subject in this case) and allows them to try, it would be nice to show them some hint of failure in case the service returns an error.

Steps to reproduce

Enter a MUC room where you are not allowed to change the subject, attempt to change the subject.

The room will respond with an error, most likely auth / forbidden as in example 87 of XEP-0045

Eg Prosody will reply like this:

<message from="room@rooms.example.com" type="error" to="me@example.com/gajim.NMDGIQG8" id="f93be961-2d60-4878-9046-c972a19f3816">
  <error type="auth">
    <forbidden xmlns="urn:ietf:params:xml:ns:xmpp-stanzas"/>
  </error>
</message>

Observe that this error appears in the XML console but not in the UI.

Software versions

Gajim 0.16.5-c8fb88f45185

Edited Jun 01, 2018 by Zash