tsweb: make BucketedStats not track 400s, 404s, etc

Updates tailscale/corp#18687

Change-Id: I142ccb1301ec4201c70350799ff03222bce96668
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