Fix hanging erasure writes (#12253)

However, this slice is also used for closing the writers, so close is never called on these.

Furthermore when an error is returned from a write it is now reported to the reader.

bonus: remove unused heal param from `newBitrotWriter`.

* Remove copy, now that we don't mutate.

cmd/erasure-encode.go

@@ -20,7 +20,6 @@ package cmd
 import (
 	"context"
 	"io"
-
 	"sync"
 
 	"github.com/minio/minio/cmd/logger"