COMMITTED: SE_InterruptCasting

[Caryatis]

Kinda minor one but seemed easy to bang out, used in the bard AA Cacophony and some late expansion AEs.

code...
spell effects.cpp - line 2818 - add this

case SE_InterruptCasting:

spell effects.cpp - line 3288 - add this

		case SE_InterruptCasting:
		{	
			if(IsCasting())
			{
				if(MakeRandomInt(0, 100) <= spells[spell_id].base[i])
				{
					InterruptSpell();
				}
			}
			break;
		}