Fix issue where chainSampleIterator can obscure errors

[charleskorn]

If any iterator in a chainSampleIterator has a sample after the desired timestamp when Seek() is called, Seek() will not return ValNone.

If some iterators have failed (ie. calling innerIterator.Err() returns a non-nil value) and others have not, this behaviour may hide errors from those iterators: callers of Seek() will not receive ValNone and therefore won't know to check chainSampleIterator.Err().

Next() is susceptible to a similar issue if Next() is not called until the stream is completely exhausted.

This PR upstreams the change from grafana/mimir-prometheus#540, which has already been tested in Mimir.

[bwplotka]

Thanks! LGTM, just a few readability nits if you don't mind (killing else - my life mission 🙈 )

Rationales: https://thanos.io/tip/contributing/coding-style-guide.md/#control-structure-prefer-early-returns-and-avoid-else

[charleskorn]

@bwplotka any chance you could take another look at this? I've addressed your feedback.

[bwplotka]

Epic, thanks!