pool: Don't count `context canceled` as error 

[`handleError`](https://github.com/TrueCloudLab/frostfs-sdk-go/blob/cf64ddfb143c8ac58907c4374d8dc25935da32c9/pool/pool.go#L996) func shouldn't consider `context canceled` as error to protect the view of storage node healthiness from user actions.

Related https://github.com/TrueCloudLab/frostfs-s3-gw/issues/35
