feat: improve `registerFallbackValue` error messaging about mocktail HOT 3 CLOSED

Hi! I just stumbled upon the:

Bad state: A test tried to use `any` or `captureAny` on a parameter of type `UserState`, but
registerFallbackValue was not previously called to register a fallback value for `UserState`

To fix, do:

\```
void main() {
  setUpAll(() {
    registerFallbackValue<UserState>(UserState());
  });
}
\```

If you cannot easily create an instance of UserState, consider defining a `Fake`:

\```
class UserStateFake extends Fake implements UserState {}

void main() {
  setUpAll(() {
    registerFallbackValue<UserState>(UserStateFake());
  });
}
\```

I thought that my use case and/or thought process will help you improve the error message.

I am using both mockito and bloc_test in my tests, but I have import on mocktail (aliased mocktail) in my test file where I need mocked cubit. I have this code:

BlocProvider<UserCubit>(create: (_) {
  final mock = MockUserCubit();
  mocktail
      .when(() => mock.state)
      .thenReturn(UserState.loadSuccess(user0));
  return mock;
}),

This is what I'd do in Mockito. But this throws the error above. I'm not sure why does this happen nor why do I need to call this registerFallbackValue.

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Hi @Albert221 👋
Sorry for the inconvenience! This is currently working as expected as described in #42 (comment). Let me know if that helps 👍

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Closing this because this is working as expected. In the above example, when is evaluated before setUpAll which means that registerFallbackValue hasn't been called yet. In general, I don't recommend using when outside of setUp or test because it will likely result in unexpected behavior since that code will be evaluated prior to any test setup or execution.

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