ExportSnapshot should validate the source snapshot before copying files

Running ExportSnapshot with default parameters will copy the snapshot to a target location, and then use verifySnapshot to validate the integrity of the written snapshot. However, it is possible for the source snapshot to be invalid which leads to an invalid exported snapshot.

We can validate the source snapshot before export.

By default, we can validate the source snapshot unless the -no-target-verify parameter is set. We could also introduce a separate parameter for -no-source-verify if an operator wanted to validate the target but not validate the source for some reason, to provide some amount of backwards compatibility if that scenario is important.