Prevent already failed objectives from being marked as completed

The same change was not and must not be done in the reverse case. It
should be and is possible to mark an already completed objective as
failed.

Consider a case where a player is supposed to capture, but not destroy,
a building.  He captures it, the objective is complete.  However, later
in the game that building gets destroyed. In this case, the objective
should be changed to failed.
Add more functions for objectives management to Lua API
Update existing maps
Merge pull request #6494 from obrakmann/objectives-fine-tuning

Objectives fine-tuning