Change the block's signature of `inSavePoint:`.

This is open to discussion, but
```
[db inSavePoint:^BOOL{
	return [self performSomethingThatFailsWithNO];
}];
```
looks tastier than
```
[db inSavePoint:^(BOOL *rollback){
	BOOL result = [self performSomethingThatFailsWithNO];
	if (!result) {
		*rollback = YES;
		return;
	}
}];
```