The file-based cache implementation was not reporting the correct error when
attempting to load a missing column key. Make it do so, and update the tests to
cover that case.
Updates #12639
Change-Id: Ie2c45a0a7e528d4125f857859c92df807116a56e
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>