qemu-nbd: Clean up "Failed to load snapshot" error message

qemu-nbd: Clean up "Failed to load snapshot" error message

bdrv_snapshot_load_tmp() sets an error and returns -errno on failure.
We report both even though the error message is self-contained.  Drop
the redundant strerror().

While there: setting errno right before exit() is pointless, so drop
that, too.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-Id: <1450452927[email protected]>