health: don't show login error details with context cancelations

Fixes #12991

Change-Id: I2a5e109395761b720ecf1069d0167cf0caf72876
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