Previously, its other return value was used even when it returned an
error. This is awkward and goes against the convention. It also could
have resulted in a nil pointer dereference when an error was returned
because of an unknown pool type. This changes the unknown pool type
error to a panic (since the pool types are hardcoded at call sites and
must always be "push" or "pull"), and returns a "found" boolean instead
of an error.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
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