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).

[JoshuaJB]

In 17587:

Map more specific messages from the Gloox documentation to registration results and stanza errors in the lobby code. Also, don't request translation of more arcane errors. Fixes #3047

[Josh]

Turns out that all the stanza errors were also improperly mapped. I fixed both mappings and narrowed the number of translated errors. Untranslated errors should be displayed like

Error. (The server exploded!)

Where Error. is translated into the user's native language and the error, in English, is contained by the parentheses.

[Imarok]

refs #3771

[elexis]

In 19575:

Revert the wrong "insufficient permissions to remove an account" error message from rP17587 to the previous "Registration forbidden" which is shown when lobby registrations are disabled or restricted to some IP range.

Differential Revision: ​https://code.wildfiregames.com/D319
Refs #3047
Reviewed By: scythetwirler, leper