Fix the error message when failing to restore the snapshot.

Fix the error message when failing to restore the snapshot.

The SnapBuildRestoreContents() used a const value in the error message to
indicate the size in bytes it was expecting to read from the serialized
snapshot file. Fix it by reporting the size that was actually passed.

Author: Hou Zhijie
Reviewed-by: Amit Kapila
Backpatch-through: 16
Discussion: http://postgr.es/m/OS0PR01MB5716D408364F7DF32221C08D941FA@OS0PR01MB5716.jpnprd01.prod.outlook.com