rest/storage: Remove racy LastError usage (#8817)

instead perform a liveness check call to
verify if server is online and print relevant
errors.

Also introduce a StorageErr string error type
instead of errors.New() deprecate usage of
VerifyFileError, DeleteFileError for gob,
change in datastructure also requires bump in
storage REST version to v13.

Fixes #8811

cmd/storage-interface.go

@@ -28,7 +28,6 @@ type StorageAPI interface {
 	// Storage operations.
 	IsOnline() bool   // Returns true if disk is online.
 	Hostname() string // Returns host name if remote host.
-	LastError() error
 	Close() error
 	SetDiskID(id string)