Propagate send errors for some of the discv5 functionality.

[kdeme]

The send proc currently just logs errors through a async callback.

For some procs that use this it doesn't matter much, e.g. sendWhoareyou:

• Here you wouldn't care much that it didn't manage to send. Not much you can do about it if it is a network issue. You would just be able to delete the handshake from the table immediately (or not add it in the first place). It would be similar for Nodes and Pong replies. Retrying sounds useless because by the time it works again timeouts are likely to have passed.

For other procs it might matter more:

• ping: Important on (re)validation of nodes in the routing table. You want to be able to differentiate between e.g. a send failure because of the own node network connection, or because of a lost packet.
• findNode: Perhaps a little less important, but the lookup could stop early instead of trying many findNodes that will all fail.

So basically, send would have to propagate the error, e.g. through Future[Result[...]].
Other option would be to stop the loops but that sounds more complex.