Improve error handling in Querier

[sGy1980de]

Currently, the querier component always raises an error, if there was a network issue or an error message in the response of the sub-service. This cannot be distinguished in the current implementation, and therefore no fine-grained error handling is possible.

The component should be reworked, so it will ultimately distinguish between these two types of error, and allow handling the latter case in the merger according to the GQL spec.