20520: Fix unreleased mutex on error importing SSH key.

20520: Fix unreleased mutex on error importing SSH key.

Any error listing or importing keys (which, luckily, only happens the
first time a arvados-dispatch-cloud process creates a new instance)
would cause Create() call to fail, and cause all subsequent Create()
calls to hang forever until the service is restarted.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curii.com>