cmd/derper: in manual cert mode, don't discard error from VerifyHostname

Updates #3701

Change-Id: If8ca5104bd8221c99cc390ca49ee3401aff09b62
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f