Fix addnum

hal/instructions.go

@@ -135,9 +135,22 @@ var InstructionJumpNeg = &Instruction{
 	},
 }
 
-var InstructionAddnum = &Instruction{
+var InstructionAdd = &Instruction{
+	Name: "ADD",
+	ExecuteWithOperand: func(module *Module, operand float64) error {
+		index := int64(operand)
+		if _, ok := module.programStorage[index]; !ok {
+			return fmt.Errorf("index %d does not exist in program storage", index)
+		}
+
+		return nil
+	},
+}
+
+var InstructionAddNum = &Instruction{
 	Name: "ADDNUM",
-	ExecuteWithOperand: func(module *Module, operand float64) {
+	ExecuteWithOperand: func(module *Module, operand float64) error {
 		module.accumulator = module.accumulator + operand
+		return nil
 	},
 }