While other log-levels are not currently used in containerd itself,
they can be returned by `GetLevel()`, and are accepted (no error) by
`SetLevel()`. We should either accept those values, or produce an
error (in `SetLevel()`), but given that there's other ways to set the
log-level, we should probably acknowledge that this package is a transitional
package, and still closely tied to logrus (for the time being).
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
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