Improve resilience and error handling of retrieving pastes for an account

[akamsteeg]

When getting the pastes for an unknown account, the 404s and empty responses are not correctly handled. This leads to confusing exceptions about failed JSON deserialization etc.

Solution approach: Correctly handle 404s and empty responses and return an empty collection in that case.

[akamsteeg]

Unfortunately the fix is a breaking change because we now return an empty collection again instead of throwing an exception.