Classic 1.12 SMSG_CAST_RESULT uses an enum starting at 0=AFFECTING_COMBAT
(no SUCCESS entry), while WotLK starts at 0=SUCCESS, 1=AFFECTING_COMBAT.
Without this override, Classic result codes were handled by TBC's
parseCastResult which passed them unshifted, causing result 0
(AFFECTING_COMBAT) to be silently treated as success with no error shown.
This applies the same +1 shift used in parseCastFailed so all Classic
spell failure codes map correctly to getSpellCastResultString.