Post-transfer Verification: Print out clearer message if verification fails

[abiswal2001]

Setup: Start a long running transfer with skyplane cp. Then, delete some objects in the portal midway through the transfer.

Once the post-transfer verification completes, the following error is printed:

At the end of this error, we are informed that 5 objects failed verification. This message should be clearer, and we should print the names of these objects that failed.