swarm: return a more meaningful error when dialing QUIC draft-29

[marten-seemann]

No description provided.

[marten-seemann]

Nice one, thanks @Jorropo!

[Jorropo]

LGTM, you need to go fmt since I used space instead of tabs in my suggestion.
Thx

[Jorropo]

Also ErrQUICDraft29 (type) is not an expression, up to you, you could do that instead:

var ErrQUICDraft29 errQUICDraft29

type errQUICDraft29 struct{}