Add NPC vertical object detection tweak

Closes #141
Make GMST negative value check Oblivion-specific
Update message for negative GMST values

The old message implied that only float GMSTs couldn't be negative. The
negative-value check doesn't discriminate between GMST types, so the new
message reflects that.
Merge branch 'trira-issue_141' into dev
Add missing data files to skyrim_const.bethDataFiles

Closes #143