Core/Spells: Fix Ebon Plague and Wandering Plague

Ebon Plague fix by @Unholychick
Wandering Plague fix by @tobmaps
Thanks to @maxxx for Ebon Plague correction.
Core/LootMgr: improved logs of missing *_loot_templates
Merge pull request #13313 from ShinDarth/335_fix

Improve logs of missing *_loot_templates
Core/ObjectAccessor: don't use singleton when calling static methods
Merge pull request #13334 from Exodius/master

Core/Spells: Fix Ebon Plague and Wandering Plague