util/groupmember: fail earlier if group doesn't exist, use slices.Contains

Noticed both while re-reading this code.

Updates #cleanup

Change-Id: I3b70f1d5dc372853fa292ae1adbdee8cfc6a9a7b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f