gmod_game: Added duel animation constant

Revision 854
Added duel animation constant
Improved IsValid, string.Explode and string.Replace
Fixed some functions having undesired return values
Added error checking to string __index metamethod for easier debugging Changed Files:
[ul]
[li]trunk/gamemodes/base/entities/weapons/weapon_base/sh_anim.lua [/li][li]trunk/lua/includes/extensions/string.lua [/li][li]trunk/lua/includes/util.lua[/li][/ul]

Committed By Alexander Overvoorde