missing error check in appendRowsBlocks

## Observed

appendRowsBlocks may return nil even if an error happened.
in our project we use Append with blocks, so it's getting into the appendRowsBlocks codepath. We observe incomplete streams being appended without errors.

## Expected behaviour

Return error if a mid-stream error happens in the appendRowsBlocks codepath.

In
https://github.com/ClickHouse/clickhouse-go/blob/a1ee16be2f6db74a3d2bd3c6f0dada2e0725871f/conn_batch.go#L138
when returning after looping on r.Next()
https://github.com/ClickHouse/clickhouse-go/blob/a1ee16be2f6db74a3d2bd3c6f0dada2e0725871f/conn_batch.go#L162

return `r.Err()` instead of `nil`.

Start in conn_batch.go at appendRowsBlocks, especially the return after the loop over r.Next() around lines 138 and 162. Confirm that a mid-stream failure is surfaced through r.Err() rather than nil; done means incomplete block appends return the encountered error.

Written by the indexing model from the issue text.