ARTRest handles JSON error incorrectly

[maratal]

Initially was reported in flutter - https://github.com/ably/ably-flutter/issues/583

When server responds with a http error in JSON format, library assumes it's a dictionary with an error sub-dictionary. error can be just a string.

You can reproduce this error by running test__014__RestClient__client_should_handle_error_messages_in_plaintext_and_HTML_format test with https://api.restful-api.dev/objects/<not_existing_object_id> url.

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