Refactor contentLookup/traceContentLookup + add failed requests to traceContentLookup
When currently doing a `traceContentLookup`, if the request failes (e.g. uTP connection set-up) this is not available in the returned responses of the trace. This makes it not immediately clear from the trace result that these nodes did get contacted by the node.

This should be added, here: https://github.com/status-im/nimbus-eth1/blob/master/fluffy/network/wire/portal_protocol.nim#L1427

However, in the current implementation we have no access to the information of the target node at the failure point.
One way would be to provide that in the error result, but that is not so clean. It would probably better to refactor the code to have some worker functions that deal with the `FindContent` requests and also add the results to the trace data.

