tsweb: handle panics in retHandler

We would have incomplete stats and missing logs in cases
of panics.

Updates tailscale/corp#18687

Signed-off-by: Maisem Ali <maisem@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f