Gracefully deal with older savegames failing to load in build 19

[Hans Joachim Desserud]

First of all, I know: we replaced the persistence layer, backwards compatibility is broken, older save games will NOT work in build 19. We can include this information in announcements, the release notes etc, but at the end of the day people will attempt to load their older save games after upgrading. And as of today, attempting to load older save games will blow up, see example error message:
 Reading Map Objects ... terminate called after throwing an instance of 'Widelands::Map_Object_Descr::Animation_Nonexistent'

Program received signal SIGABRT, Aborted.

We need to deal with this in a more graceful manner. Usually when games fail to load I get an dialog with an error message informing me what went wrong. (Btw, I wonder why this case doesn't trigger such a dialog, is it some corner case which hasn't been caught in the past?) We should probably use such a dialog to explain that this is an older game which will be unable to load in build 19 due to the various changes, and allow the user to return to the menu.

I haven't marked this as Critical because there is no rush, but we should have fixed this before the final release of build19.

Widelands r6914.
(I've attached an example save game. It's called build16 which probably makes it sufficiently old :P)

[SirVer (sirver)]

In r6926 I increased the version of the elemental data packet and all other data packets now throw an error. This means that all old savegames will be hidden in the GUI and that a meaningful crash will be displayed when one is loaded from the cmdline.

[GunChleoc (gunchleoc)]

Fixed in build19-rc1.