Make sure to handle FaultyDisks in listing ops (#6204)

Continuing from PR 157ed65c35

Our posix.go implementation did not handle  I/O errors
properly on the disks, this led to situations where
top-level callers such as ListObjects might return early
without even verifying all the available disks.

This commit tries to address this in Kubernetes, drbd/nbd based
persistent volumes which can disconnect under load and
result in the situations with disks return I/O errors.

This commit also simplifies listing operation, listing
never returns any error. We can avoid this since we pretty
much ignore most of the errors anyways. When objects are
accessed directly we return proper errors.

cmd/tree-walk.go

@@ -86,7 +86,7 @@ func filterMatchingPrefix(entries []string, prefixEntry string) []string {
 }
 
 // "listDir" function of type listDirFunc returned by listDirFactory() - explained below.
-type listDirFunc func(bucket, prefixDir, prefixEntry string) (entries []string, delayIsLeaf bool, err error)
+type listDirFunc func(bucket, prefixDir, prefixEntry string) (entries []string, delayIsLeaf bool)
 
 // A function isLeaf of type isLeafFunc is used to detect if an entry is a leaf entry. There are four scenarios
 // where isLeaf should behave differently:
@@ -141,16 +141,8 @@ func doTreeWalk(ctx context.Context, bucket, prefixDir, entryPrefixMatch, marker
 			markerBase = markerSplit[1]
 		}
 	}
-	entries, delayIsLeaf, err := listDir(bucket, prefixDir, entryPrefixMatch)
-	if err != nil {
-		select {
-		case <-endWalkCh:
-			return errWalkAbort
-		case resultCh <- treeWalkResult{err: err}:
-			return err
-		}
-	}
 
+	entries, delayIsLeaf := listDir(bucket, prefixDir, entryPrefixMatch)
 	// When isleaf check is delayed, make sure that it is set correctly here.
 	if delayIsLeaf && isLeaf == nil {
 		return errInvalidArgument