Print storage errors during distributed initialization (#6441)

This commit will print connection failures to other disks in other nodes
after 5 retries. It is useful for users to understand why the
distribued cluster fails to boot up.

cmd/storage-interface.go

@@ -27,7 +27,9 @@ type StorageAPI interface {
 
 	// Storage operations.
 	IsOnline() bool // Returns true if disk is online.
+	LastError() error
 	Close() error
+
 	DiskInfo() (info DiskInfo, err error)
 
 	// Volume operations.