Fixed AsyncStorage `exists` not throwing errors

[kitwtnb]

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Current existsObject() returns true or throw error, not returning false.
As per StorageAware, existsObject() should not throw an error.
Fix existsObject() to return false instead of error.

[3lvis]

Hi @kitwtnb,

Would you be so kind on adding a tests that reproduces the error as well to avoid regressions?

[kitwtnb]

Changed Result<Bool, Error> to Bool and removed throws.
So we could express by type that the previous problem does not regression.

Do I need to test separately from these?

[3lvis]

Can anyone else verify if this breaks anything for them?

[kitwtnb]

I understand what you are trying to say.

This test originally reproduced the problem.
I expect it to throw an Error instead of false if the object does not exist.
https://github.com/hyperoslo/Cache/pull/325/files#diff-360238a58739325a7ce97d0a648b466797db4442b6cae0592f1bb2e26c0f97d7L90