refactor: do not swallow non-backend errors in datasets()

[mortenpi]

datasets() will filter out any dataset JSON objects it can not parse, just to gracefully degrade when there is something weird in the backend response, but only for some datasets. However, currently it also hides e.g. any coding errors in the Dataset() constructor, because we catch things unconditionally.

This changes Dataset to consistently throw JuliaHubError when it encounters a backend response it can not parse, and datasets() will then only filter those datasets. Any other errors will get rethrown, making datasets() itself throw, directly exposing non-backend issues to the caller.

It also adds direct test coverage on the error handling in Dataset(::Dict) calls, which in turn required some slightly refactoring of the mocking code (to reuse the bit that constructs the dataset JSON objects).