Add testnet3 dnsseed testnet-seed.bitcoin.schildbach.de
Use Errorf when using formatting directives.

Found by go vet.
IMPORT PATH MODIFICATION CHANGESET
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Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.