Fixed terminating due to a corrupt savegame.

When in the load dialog a corrupt savegame was selected the game would
terminate due to an unhandled exception. If the savegame was the newest
savegame it was no longer possible to open the dialog at all.

(Fixes bug #20290.)

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56267 75b3775d-3801-0410-9e12-e2e43121521d
Catch an exception by const reference.

A const reference is preferred over a normal reference, also minor
coding style fixes.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56268 75b3775d-3801-0410-9e12-e2e43121521d