The most important part is fixing resize handling - call size_func
before data_func, but after tar gets initial data (and output file
size).
But other than that, it makes the process a little faster.
QubesOS/qubes-issues#1214
Opt for a simple one-liner error messages, instead of meaningless stack
trace (it's most of the time about qubesd responding with error, so the
stack trace of actual problem is elsewhere).
