[ffigen] Nicer error messages when ObjC Blocks are invoked after deletion.

Currently if an ObjC block is invoked by native code after it has been deleted, we get a null assertion failure here, because `block.ref.target.address` is some junk value:

```Dart
void _ObjCBlock_ffiVoid_CMPedometerData_NSError_closureTrampoline(
ffi.Pointer<_ObjCBlock> block,
ffi.Pointer arg0,
ffi.Pointer arg1) =>
_ObjCBlock_ffiVoid_CMPedometerData_NSError_closureRegistry[
block.ref.target.address]!(arg0, arg1);
// ^ NPE here
```

Instead we should throw an exception with an error message explaining that this is probably caused by the block having been deleted.