virsh: blockcopy: Report error if the copy job fails

virsh: blockcopy: Report error if the copy job fails

When the block job would fail while watching it using the "--wait"
option for blockcopy, virsh would rather unhelpfully report:

$ virsh blockcopy vm hdc /tmp/raw.img --granularity 4096 --verbose --wait

Now in mirroring phase

Add a special case when the block job vanishes while waiting for it to
finish to improve the message:

$ virsh blockcopy vm hdc /tmp/raw.img --granularity 8192 --verbose --wait
error: Block Copy unexpectedly failed