testcontrol: replace panic with error

I have seen this once in the VM test (caused by an EOF, I believe on
shutdown) that didn't need to cause the test to fail.

Signed-off-by: David Crawshaw <crawshaw@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