Improve clarity of XMPP error messages

[Itms]

When using the game lobby, XMPP sometimes throws error messages, for instance "Registration not allowed". I would be interesting to add code to find the exact reason of such errors, so that users can find a way to fix them. (Currently, the most confused people seem to be translators trying to find good translations of those messages :P)

[leper]

Restating what I answered to some translators (and as the one noticing that we didn't translate any of the possible (though most are very unlikely, unless you set up the lobby server in a specific way) error messages and making them translatable in the first place):

Connection error (%(untranslated_error_message)s) would be a relatively nice way to handle this. In addition to that some error messages should be properly translated (eg Forbidden).

[Josh]

It looks like all the registration errors are improperly mapped to messages. I'll fix them using the official mappings from the Gloox documentation (​https://camaya.net/api/gloox/namespacegloox.html#a11d4a3a83daee22120b47d3fe3deb974).