MAI test scenarios: fight_on_without_leader -> defeat_condition

Necessary due to a recent change how the victory conditions are
evaluated.
refactor is_observer out of check_victory

This is a minor change to victory_when_enemies_defeated condition.

Old logic: If there are no not defeated sides which are enemies
of eachother (i.e. there is still a battle going on),
BUT victory_when_enemies_defeated = false, and either we found
a not defeated local human player, or we are a multiplayer observer
(there are no locally controlled sides), then don't end the level.

New logic: If there are no not defeated sides which are enemies,
BUT victory when enemies defeated = false, and EITHER we found a
not defeated local human player, OR there is a not defeated remote
human player AND no local human side assigned at all,
then don't end the level.

I think this more precisely the behavior we want. is_observer
doesn't belong in this function, and with the introduction of
defeat_condition, UMC will pretty much always be able to get the
behavior they want, so I'm much more comfortable to introduce this
change now than I would be otherwise.
update changelog
update release notes
fixup error logging, catch parsing exceptions in editor, -p option

Conflicts:
	src/savegame.cpp
Catch config::error exception which could be thrown by parser.

I'm not sure if this is necessary as it is not reported, but
worst case is that not doing this could cause malformed
scenario files in the editor folder to crash wesnoth while
browsing in mp_create.
update changelog
Fixed Li'sar not carrying over correctly
cleanup check_victory in ai actions

do_execute which calls run_in_synced_context can throw end_level
exception because run_in_synced context calls check_victory. So we don't
need the additional check_victory in action_result::execute anymore.
remove check_victory in play_human_turn

we don't need these because we call check_victory at the end of the
synced_context.

I removed them because check_victory assumes that "this code is run
rarely" which is not the case if we call it there.