Specify that UDP DNS errors are emitted to the socket's 'error' listeners

[mcollina]

See #2900 and #4846

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[bnoordhuis]

Arguably more appropriate as a dgram.Socket#send() note?

[mcollina]

It is a note to dgram.Socket#send(), but it is after the note about datagram size. I think the one about datagram size is more important, so I put this one after that.

[bnoordhuis]

I've decided against landing the patch for now. I've reopened #4846 because I think we need to look at the API again; I've been playing around with it and I've got to admit the current behavior is pretty bad.