disconnected returns, an unexpected error to List() returning 500s (#18959)

provide the error string appropriately so that the
matching of error types works.

Also add a string based fallback for the said error.

cmd/storage-rest-client.go

@@ -148,6 +148,8 @@ func toStorageErr(err error) error {
 		return errDiskOngoingReq
 	case grid.ErrUnknownHandler.Error():
 		return errInconsistentDisk
+	case grid.ErrDisconnected.Error():
+		return errDiskNotFound
 	}
 	return err
 }

internal/grid/connection.go

@@ -1374,7 +1374,7 @@ func (c *Connection) handleDisconnectClientMux(m message) {
 		if m.Flags&FlagPayloadIsErr != 0 {
 			v.error(RemoteErr(m.Payload))
 		} else {
-			v.error("remote disconnected")
+			v.error(ErrDisconnected)
 		}
 		return
 	}

internal/grid/grid.go

@@ -30,7 +30,7 @@ import (
 )
 
 // ErrDisconnected is returned when the connection to the remote has been lost during the call.
-var ErrDisconnected = errors.New("remote disconnected")
+var ErrDisconnected = RemoteErr("remote disconnected")
 
 const (
 	// minBufferSize is the minimum buffer size.