feat(sdk): support mocking of error responses

[lklimek]

Issue being fixed or feature implemented

We are not able to run offline tests (without network connectivity, eg. errors) as we don't support serialization of errors.

What was done?

Implemented Mockable for Result<T,E> and for DapiClientError.

How Has This Been Tested?

Locally, generated test vectors.

Breaking Changes

None

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone