Show filename & line number for file parsing errors

For most instances of file parsing, I've also added error checking for
the validity of each parsed key. This should help track down obsolete
stuff in mods.
Merge pull request #1152 from dorkster/logging

Show filename & line number for file parsing errors
Remove unused code from NPC

level is parsed, but isn't used anywhere
stock, vox_intro, vox_quests and dialog are member variables
Merge pull request #1153 from mazayus/fixes

Remove unused code from NPC
Prevent items being equipped to wrong slots when no slot is available

When item ids are the same, we need to make sure that source quantity
+ destination quantity does not exceed the max quantity for that item.
Otherwise, ItemStorage::add() will try to place leftover quantity in the
first empty equipment slot, regardless of type.

Closes #1137