fix a segfault when a redscreen error is closed. fixes #2466.

Thanks to @pgimeno for reporting this, tracking it down and fixing it.
show a recoverable error message if game load fails from the main menu
Merge remote-tracking branch 'johnbartholomew/survive-invalid-savegames' into survive-invalid-savegame
use a MultiLineText for long error messages
Merge branch 'survive-invalid-savegame'