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Rather than pushing errors back down to lower layers, propagate the
errors upward.

This commit adds a `ReceiveError` method to BitSwap's network receiver.

Still TODO: rm the error return value from:

    net.service.handler.HandleMessage

This is inspired by delegation patterns in found in the wild.

misc:
* test network client getting more than max
* test for find providers
* rename factory method
* local network
* misc test improvements
* test bitswap get block timeout
* test provider exists but cannot connect to peer
* test sending a message async over local network